As far as booking, yes, it's an excursion and can be pre-booked 60 days out. There is an upper weight limit, can't think of what it is. I know many adults go tandem, so I'd guess it's over 300 pounds.
As far as how it works, you go out on a small fishing type boat with a platform on the back. You hook into a harness thing with a parachute and lift off that platform. They ask you if you want to get your feet wet. If so, they'll touch you down to the water otherwise, they just reel you back in to the platform. You don't have to swim, and you won't get wet unless you ask.