An inflatable rubber boat includes a flat bottom fabric member formed of a fabric having a front V-cut and two lateral sides, the two border edges of the front V-cut being seamed together to form a forwardly upwardly curved front end edge of the flat bottom fabric member, an elongated main inflatable airbag member fastened to the forwardly upwardly curved front end edge and two lateral sides of the flat bottom fabric member, a floorboard member having a body fitting over the flat bottom fabric member and a pointed arch-shaped rib obliquely upwardly extended from the front side of the body and fastened to the contained angle defined between the upwardly curved front end edge of the flat bottom fabric member and the elongated main inflatable airbag member, and a supplementary inflatable airbag member longitudinally set between the floorboard member and the flat bottom fabric member and inflated to stretch the flat bottom fabric member into shape.