This may sound crazy but I really think the quickest way is to walk. The boat is less tiring but it moves slow and will stop at Boardwalk, YC/BC, Swan/Dolphin and finally MGM. I guarentee you can get there faster walking. You can exit the front gate and get a bus from Epcot to MGM but that also will take a while. IF you are near the International Gate when you decide to go to MGM then definatley take the boat or if your up for it, take a walk.