PURPOSE: To manufacture boats having large strength and a great bouyancy at a low cost by forming a shell plating portion and inner bottom plating portion of a hull in a blow molding method or the like using molds and as well by joining both portions leaving spaces disposed therebetween.

CONSTITUTION: A shell plating portion 3 and an inner bottom plating portin 4 of a hull are integrally formed in a blow molding method, a vacuum molding method, a vacuum molding method, a vacuum-blow molding method or the like with the use of molds. As occasion demands, the shell plating portion 3 is formed with reinforcing ridges 16 and edge portions 6, and the inner bottom plating portion 4 is formed with a seat extension 5, reinforcing recesses 17 and inner bottom plates 4. Both plating portions 3, 4 are jointed together, leaving a space disposed therebetween, so that buoyancy is made great. The shell plating portion 3 and the inner bottom plating portion 4 may be massproduced at a lower cost by using mold dies, respectively.