You might look around and see if someone's got a back door link to "yostwerks". Tom Yost has (had?) several SOF (skin on frame) kayak designs and several were covered with clear vinyl, so you could see through the bottom. True enough what someone said, there is little room between your legs while sitting in a kayak to look down and also what someone else said, the clear material tends to fog up pretty quickly so you can't see anything anyway.

Btw, polycarbonate (lexan) is pretty expensive and heavy. While it is heat form-able it tends to go from clear to cloudy when you heat it specially when the material is thick enough for a kayak's structure. Might look into polycarbonate film, it's still clear and strong enough for a skin over a frame, would still need to be heat formed at the edges but since it's thin it won't need as much heat. The problem is you need to heat the entire length all at once to form it, takes some serous tooling. It might be ok for production work, it's expensive for a one off boat.

Bill H.