The walk from MGM to BW would be about 15 minutes, and the BW to Epcot walk us maybe 10. If you're leaving the Int'l gateway, you go over the little bridge that goes left (follow signs to BW), walk around the boardwalk shops and restaurants until you get to the bicycle rental. You can then go left on the path and under the building toward the pool. Walk around the pool to the right, and down a path and stairs that lead to the canal. You'll walk along the canal for a while - under a bridge, etc. You'll see MGM across the water on your right. You'll walk past the bus parking area and then turn right into the entrance.
There are ferryboat docks at Epcot, BW, MGM, Swan/Dolphin, and BC/YC. I forget what in what order the boats stop at the docks, but you could always ask and find out. You could go from Epcot to MGM on the boat, but it will probably make a few stops on the way. Because of that, the walk is probably faster - but if it's hot or you're tired - you might want to just sit on the ferry.