If you're healthy, the weather is reasonable, and you're in World Showcase, the fastest (and my favorite) way from Epcot to MGM is to walk. There is a walking path that begins at the end of the Boardwalk near the Swan. You can take a shortcut through BWV to save a little time.
It's about 1 1/4 miles from the International Gate to MGM entrance. About 20-30 minutes for most people. (As little as 15 minutes for fast walkers!)
The boat is best for a lazy, unhurried trip. You can easily wait 15-20 minutes for a boat, and then it makes 3 stops on the way to MGM (BW, YC, S/D). If I started walking at the same time you left in a boat, I'd be waiting for you at the dock.
If you're in Future World at the time, or if anyone is not up to the hike, going out the main entrance and catching a bus is probably the best bet.