Every time we have rented watercraft at WDW, we have always been told that we are not to dock anywhere, and we are to return our watercraft back to the marina from which we rented them from no exceptions.
Discovery Island was a great place, but it has been closed for years now. No one is allowed to dock there and if they attempted to do so, they would surely be caught by the Shore Patrol boats that patrol Bay Lake. The island is off-limits to everyone but specific WDW Cast Members.