I seem to recall that the Dolphin boat rentals are in their little private lagoon - so it's just paddle boats and the like.
There is no place for a marina on the other side since that's where the WDW boat docks.
You can always walk down to Yacht Club and rent motorized boats (water mice) there - you can boat in Cresent Lake and over to Studios (can't dock, of course). It's a small area, and only good for sightseeing and putting around.
If you really want to open 'er up - go over to the MK resorts and rent a water mouse there - Between Seven Seas Lagoon and Bay Lake - there's plenty of room to zip around!!!
-Kim