Can't get there by boat - they are on two different bodies of water.
If MGM is open, you could take the bus to MGM, then a boat to the Boardwalk, or even just walk (about 10 -15 minutes from MGM to Boardwalk - there's a path along the water). Or, if you plan your evening at the Boardwalk to follow a day at Epcot, you could take the boat or walk from the International Gateway at Epcot to the Boardwalk.
Otherwise, you'll have to transfer busses to get there & back. You could take the bus (or the boat if it's still running - not sure) from POFQ to DTD, and then get a bus to the Boardwalk.
