you can make any shape out of fiber glass, thats how the the rice rocket boys make a lot of the shapes for their car, its a simple thing actually, with a wire frame and a felt blanket and resin or even thin wood or insulation board
depending on your structure, you can do shallow in-floor between the structures. was even offered on many 'toons as standard as a location for the anchor ropes
Hcozart,
If you are considering anything more than a shallow space for small stuff, you ought to look at this video first. There's a lot of water impact on the underside of the deck of many pontoon boats.
