There are boats that run from Epcot, stop at all the resorts on the lake (BW, YC & BC, and Swan and Dolphin), and then head to MGM - the boats then turn around and make their way back to Epcot stoping at all the resorts. It is about a 20 min trip from Epcot to MGM. If you can catch the boat at the right time, then it is always faster (and a little more relaxing) to take the boat. However, if you see the boat leaving the dock as you come up to it, you may very well want to walk, for it usually takes about another 20 minutes before the next boat heading in the same direction will come.
The walk to Epcot from BWV is only about 5 min and the walk to MGM is about 15-20 min.
By the way, the Boardwalk is a beautiful hotel!!!