You can catch the boat from the left side of the MGM exit. The boat ride is approx 25-30 minutes. It goes from MGM to Swan and Dolphin, then to Boardwalk, Yacht and Beach and then to Epcot.
I prefer to walk from MGM to Epcot. To do that stay to the right after you exit MGM. Look for the path that runs along side of the canal. That will take you over to the Boardwalk resort. Walk along the boardwalk, stop and grab a soda if you are parched and walk over to the International gateway to Epcot. The walk is nice and I always beat the boat to Epcot. Peggie