How about deepening the dished deck on a user friendly sized SUP, so deep that the top becomes the bottom, this dished area is a clear 10mm polycarbonate replaceable window, great for watching the marine life & coral as you paddle along. (but maybe some paddlers don't want to know what's under them!)
The shape of the board can be such to make the attitude of the board angled slightly stern down draining any water in the dished area aft for clear view, or if the board was buoyant enough, water could drain in a tube & scupper through the tail.
Orders taken.
Pat Pending

bum........u gota be quick these days....

Have a look at a 6 month old kayak with one of these clear bottoms in a resort...you can no longer see though the hole due to being dragged on the sand and scratched.

Anyways, I see heaps enough sea life without a window in my SUP.