To provide a water vessel or a biomass foamed board 16 which shows excellence in maintenance of a board balance and a stable directional guide due to a bending stress to a wave resistance, is composed mainly of a natural material, and recurs to nature when the board is unnecessary.

The biomass foamed board 8 is made by inputting vegetable fibers 7 in a lignocellulose solution 5 as one of foaming liquids (Fig. 5), dispersing them sufficiently, and reacting the foaming and curable liquid 6. The obtained foamed body has increased strength. Since carbon dioxide gas generated by the foaming reaction is guided and discharged to the outside by utilizing porous thick layer paper 4 (Fig. 5) in the manufacturing process, it is possible to prevent a crack due to expansion or explosion of the gas in the foamed body and as a result increase bending strength and compression strength. The surfboard 16 is made by covering the whole board with a vegetable fiber cloth 14, and coating it with a vegetable resin 15.