According to FW's website there are Sea Raycers available at the marina. No price is given though. I do not believe you need (or can even make) reservations for them, they are first come, first serve.
We went in Feb'07 and it was between $25-30 for half an hour. We were staying at ASM and we just went over for a few hours, no ressies needed. Everyone must wear a life jacket, bathing suits optional, we didn't get wet.