I would think the bus to MGM then the boat to Swan/Dolphin would be a pretty easy trip. When we're leaving the waterparks mid-day (ie when fewer buses running), we will take either the Boardwalk (where we generally stay) bus or the MGM bus, whichever comes first. The most recent time, we were pressed for time, but the MGM bus to the boat got us there in plenty of time. Easy, breezy.
Have fun!
btw, where are you off to after WDW in the rental car? just curious (or is that nosy?)