Yes you can. You cross the foot bridge between the Swan and the boat dock (going towards the Boardwalk). Turn left after the bridge and follow the walkway to MGM. It's a nice 10 minute walk along the water. It's very nice.
If you take the boat, MGM is the first stop. The boats run from Epcot, Boardwalk, Yacht/Beach Club, Swan/Dolphin, MGM then in reverse - MGM, Swan/Dolphin, Yacht/Beach, Boardwalk, Epcot.