I posted a picture when talking about the new walkway from the Boardwalk to MGM, but you can also see the boat dock at MGM down in the bottom left hand corner.
Click here for the picture. The dock is the the far north of the ticket plaza. If you where to get off the parking tram and turn right instead of left to enter the park, you would run into the boat dock eventually. The boats get very crowded after Fantasmic, and it is usually quite a bit quicker just to walk, mostly because the boats are slow, and they stop at the Swan and Dolphin before hitting the boardwalk.