As far as I know, the watercraft are rented on a half hourly basis, not length of stay (pleasem anyone, feel free to correct anything I misstate!!). The canopy boats are about $20/ half hour, same with the water mice.
You can also rent boats with drivers for more $$$.
You can drive them to DTD, but I don't know if you are allowed to disembark your boat there. I have drieven there, but never gotten off.
There is a ferry every 20 min from OKW to DD (it stops around 9:30 at night). This is a transportation feature and is a great way to get to DTD. Warning, it does get crowded around 6:30 at night from the OKW dock, so allow yourself time if you have a dinner reservation.