[001] The present patent application is a continuation-in-part of and claims priority from U.S. Patent Application Serial Number 10/883,504 filed July 1, 2004. [002] The present invention relates generally to an apparatus to convert an all terrain vehicle into an amphibious off-road vehicle, and methods of constructing and utilizing same. [003] More particularly, the present invention relates to an apparatus to convert an all terrain vehicle into an amphibious off-road vehicle by means of a one-piece shell, and methods of constructing and utilizing same. BACKGROUND OF THE INVENTION [004] Presently, there exists no apparatus for converting a four-wheel all terrain vehicle into an amphibious off-road vehicle in less than five minutes. [005] The prior, but not necessarily relevant, art is exemplified by the following United States Patents. [006] Newkirk United States Patent 4,664,051, entitled "THREE WHEELER FLOTATION CONVERSION KIT" discloses a conversion kit adapted to be retro-fitted to conventional three wheeled vehicles for floating them over water, wherein the flotation kit includes a front frame assembly having a pair of rigid, elongated front frame members semi-permanently coupled to the front fork of the three wheeler in axial alignment with one another. [007] Martinmaas United States Patent 4,744,324, entitled 'AMPHIBIOUS ALL TERRAIN VEHICLE (ATV) AND CONVERSION KIT" discloses a self-propelled all terrain vehicle which can be converted to amphibious operation by means of an amphibious conversion kit that provides rear axle extensions to mount outer rear wheels that dualize the rear wheels, and radial paddle elements on the extensions between the dual rear wheels. [008] Kadela United States Patent 4,981,188, entitled ALL TERRAIN OFF ROAD VEHICLES" discloses an all weather, all terrain, amphibious tracked vehicle for one or a small number of persons, wherein the vehicle includes a main frame structure made of strong but lightweight plastic or metal and having the general shape of an elongated enclosed box. [09] None of the conventional devices and techniques disclose or contemplate a novel and unique molded high impact polyethylene body with buoyant air pockets built in the front, back and sides formed to fit over an existing all terrain vehicle and which attaches thereto in less than five minutes as does the present invention. [010] Indeed, a desideratum of the present invention is to avoid the animadversions of the existing devices and techniques. SUMMARY OF THE INVENTION [011] The present invention provides an apparatus to convert an all terrain vehicle into an amphibious off-road vehicle, comprising, in combination: an all terrain vehicle; a buoyant unitary structure for fitting over and onto said all terrain vehicle; fastening means for releasably securing said buoyant unitary structure to said all terrain vehicle; said buoyant unitary structure including at least one hollow compartment to accommodate legs and feet of a user of said apparatus and to serve as air pockets for flotation and lateral stability; and said buoyant unitary structure including one or more air pockets therein. [012] The present invention also provides an apparatus to convert an all terrain vehicle into an amphibious off-road vehicle, comprising, in combination: an all terrain vehicle; a buoyant unitary structure for fitting over and onto said all terrain vehicle; fastening means for releasably securing said buoyant unitary structure to said all terrain vehicle; said buoyant unitary structure including a hollow compartment to accommodate legs and feet of a user of said apparatus; said buoyant unitary structure including one or more air pockets therein; marine propulsion means operatively connected to said all terrain vehicle and said buoyant unitary structure; and said marine propulsion means deriving its power from said all terrain vehicle. [013] It is a primary objective of the present invention to provide a novel and unique apparatus, as described hereinabove, to convert an all terrain vehicle into an amphibious off-road vehicle suitable for lakes and rivers. [014] Another object of the present invention is to provide such a novel and unique apparatus, as described hereinabove, which includes a molded high impact polyethylene body with buoyant air pockets built in the front, back and side portions thereof. [015] Yet a further object of the present invention is to provide such a novel and unique apparatus, as described hereinabove, which fits over an existing all terrain vehicle and attaches with four bolts enabling any all terrain vehicle to be converted to a small boat in less than five minutes using its existing wheels for propulsion and steering. [016] Another object of the present invention is to provide such a novel and unique apparatus, as described hereinabove, which comprises a sleek, lightweight, one-piece shell with storage area in back and an optional motor mount for upstream travel in streams with heavier currents. [017] Yet a further object of the present invention is to provide such a novel and unique apparatus, described hereinabove, which includes optional storage racks at the front and rear portions thereof. [018] Yet a further object of the present invention is to provide such a novel and unique apparatus, described hereinabove, which includes a hollow compartment to accommodate the legs and feet of the user of the vehicle. [019] Another object of the present invention is to provide such a novel and unique apparatus, described hereinabove, which includes a recessed motor mount in the rear portion thereof for an optional outboard engine. [020] A further object of the present invention is to provide such a novel and unique apparatus, as described hereinabove, which includes marine propulsion means to help propel said apparatus through a body of water. [021] Further objects, advantages, and features of the present invention will become more apparent to those persons skilled in this particular area of technology and to other persons after having been exposed to the detailed description as forth hereinbelow in conjunction with the accompanying patent drawings. BRIEF DESCRIPTION OF THE DRAWINGS [022] Fig. 1 illustrates a side elevational view of the major components in accordance with a first embodiment of the present invention just prior to conversion of an all terrain vehicle into an amphibious off-road vehicle. [023] Fig. 2 shows a side elevational view of the Fig. 1 components after conversion to an amphibious off-road vehicle. [024] Fig. 3 illustrates a top plan view of the assembled components as depicted in Fig. 2. [025] Fig 4 illustrates a side elevation view of a second embodiment of the present invention having a motor and a propeller mounted near the rear of the apparatus. [026] Fig. 5 shows a side elevational view of a third embodiment of the present invention having a paddle wheel linked to the rear wheels of the all terrain vehicle. [027] Fig. 6 shows a top plan view of the apparatus of Fig. 5 but shown with the fenders removed. [028] Fig. 7 illustrates a fragmentary side elevational view of a fourth embodiment of the present invention which is provided with a propeller deriving its rotating power by means of smaller tires engagable with the rear wheels of the all terrain vehicle. [029] Fig. 8 shows a schematic rear view of the Fig. 7 embodiment. [030] Fig. 9 illustrates a fragmentary side elevational view of a fifth embodiment of the present invention which is provided with a paddle wheel deriving its power from one or more smaller tires engagable with the rear wheels of the all terrain vehicle. DETAILED DESCRIPTION QF SOME PREFERRED EMBODIMENTS OF THE PRESENT INVENTION [031] With reference to Fig. 1, there is shown an all terrain vehicle 10 and an apparatus 11 for converting the all terrain vehicle 10 into an amphibious off-road vehicle 12, which is shown in Figs. 2 and 3. [032] The novel and unique apparatus 11 includes a buoyant unitary structure 13 for fitting over and onto the all terrain vehicle 10, and predetermined fastening means 14-17 and 19-22 for releasably securing the buoyant unitary structure 13 to the all terrain vehicle 10. [033] The fastening means preferably, but not necessarily, includes four upwardly extending bolts 14, 15, 16 and 17 attached to the frame 18 of the all terrain vehicle 10, and wing nuts or knobs 19, 20, 21 and 22 for releasable and threaded engagement with said bolts 14, 15, 16 and 17, respectively. [034] The buoyant unitary structure 13 includes one or more internal air pockets 23 and 25 therein, and hollow compartments 27 to accommodate the legs and feet of a user of the vehicle 12 and to serve as air pockets for floatation and lateral stability. [035] Preferably, but not necessarily, the buoyant structure 13 may also include a side air pocket 24 on both sides of structure 13. [036] The buoyant unitary structure 13 includes one or more air pockets 23, 25 and 24 or buoyant means built into the front portion, the rear portion, and/or the sides (optionally) of the buoyant unitary structure 13, which are illustrated in phantom lines in Figs. 1, 2 and 3. [037] Preferably, but not necessarily, the buoyant unitary structure 13 may comprise a molded high- impact polyethylene body 28 with buoyant air pockets 23, 25 and 24 built into the front, back and side portions, respectively, and wherein such body 28 is formed to fit over the existing all terrain vehicle 10. [038] Optionally, the buoyant unitary structure 13 preferably, but not necessarily, may include in the rear portion thereof a recessed motor mount 29 for accommodating an optional outboard engine (not shown). [039] The optional recessed motor mount 29 to accommodate an outboard engine may be used for upstream travel in streams with a heavier current. [040] It is important to understand that with the components 14-17 and 19-22 and structure 13 described hereinabove, the user is able to convert the all terrain vehicle 10 to become a small boat 12 which utilizes the existing wheels 30 of the all terrain vehicle 10 for propulsion and steering. [041] Other embodiments of the present invention will now be described hereinbelow wherein components similar to the above-described embodiment are used to designate similar parts of the additional embodiments. [042] Fig. 4 illustrates an apparatus 31 in accordance with a second embodiment of the present invention, wherein there is provided a lightweight 12-volt motor 32 for driving a propeller 33. The motor 32 is powered by the electrical battery (not shown) of the all terrain vehicle 10. [043] Preferably, but not necessarily, the apparatus 31 is provided with watertight hatches 34 and 35 with access to the air pockets 23 and 25, respectively, for storage, and which may also serve as a cooler with ice. [044] Preferably, but not necessarily, the apparatus 31 may also be provided with front and rear lights 36 and 37 which plug into the existing lights (not shown) of the all terrain vehicle. [045] Figs. 5 and 6 illustrate an apparatus 38 in accordance with a third embodiment of the present invention. The apparatus 38 is provided with a pair of paddle wheels 39 and 40 recessed into the rear portion of the buoyant unitary structure 13. Preferably, but not necessarily, the apparatus 38 is provided with fenders 41 for enclosing the paddle wheels 39 and 40. [046] The paddle wheels 39 and 40 are powered by attaching the rear wheels of the all terrain vehicle 10 to the paddle wheel shaft 42 by chains and sprockets, or belts 43 and pulleys 44. [047] Preferably, but not necessarily, the apparatus 38 is provided with front and rear lights 36 and 37 which plug into the existing lights (not shown) of the all terrain vehicle 10. [048] Preferably, but not necessarily, the apparatus 38 is provided with a moveable rudder 45 which is preferably, but not necessarily, permanently attached to the underside of the all terrain vehicle 10. The rudder 45 is controlled by a linkage 46 connected to the handlebars 47 of the all terrain vehicle 10. [049] Fig. 7 and 8 illustrate an apparatus 48 in accordance with a fourth embodiment of the present invention. The apparatus 48 is provided with a gear box 49 and shaft 50 and a propeller 51. The shaft 50 is connected to small tires 52 and 53 which are engagable with the larger tires of the all terrain vehicle 10. A lever 54 is moveable to engage or disengage the smaller tires 52 and 53 from the rear wheels of the all terrain vehicle 10 for selectively obtaining power for the propeller 51 when desired. [050] Fig. 9 illustrates an apparatus 55 in accordance with a fifth embodiment of the present invention. The marine propulsion means for this apparatus 55 includes a paddle wheel 56, one or more small rubber wheels 57, and a moveable control lever 58. [051] The small rubber wheel or wheels 57 make contact with the rear tires of the all terrain vehicle 10 by means of the control level 58 to engage and disengage, when desired. The small rubber wheel 57 engages the paddle wheel 56 which has a gear 59 mounted to mesh with a gear 60 of the small wheel 57. [052] This fifth embodiment 55 eliminates the need for attaching belts, chains, sprockets, or pulleys, and does not increase the width of the apparatus 55, and moving parts can be completely concealed. [053] Various modifications can be made in the structure and components described hereinabove. [054] Therefore, while there has been set forth above only some preferred embodiments of the present invention, it should be understood that the present invention is not to be limited or restricted in the particular embodiments described in detail hereinabove which have been set forth for illustrative purposes only. [055] The scope of the present invention is set forth in the claims which follow hereinbelow.