You can also walk from MGM to the Boardwalk if the line for the boat is too long. You exit MGM and head towards the buses. When you get to the bus info booth bear towards the bus stops on your left. As you walk along the bus stops, you will come to a path on your left that runs down to the water's edge. Just take this path to the BW. It is about a 10-15 minute walk and very pleasant if the weather is not too hot. After Fantasmic, you will see a lot of people taking this path back to the BW as the line for the boat is outrageous. We actually walked three-quarters of the way back to the BW before the boat that was at the dock left.
The boat does stop at the Swan/Dolphin, then the YC/BC, and finally at the BW before heading to EPCOT, so you can actually get to the BW much faster if you walk.