Yach & Beach Clubs are both located within walking distance of the International Gateway entrance of Epcot.
The walk takes about 5 minutes (depending on your walking speed, and what part of YC your room is in, etc).
Alternatively, there is also a boat called the Friendship Launch, which runs a loop around Crescent Lake to all of the Epcot resorts, MGM, and Epcot. The boat takes longer, but it's a pleasant ride if the weather is hot. I highly recommend taking the boat at least once during your stay, just for the beautiful views from the middle of the lake.
It's also possible to walk to MGM, but that walk is much longer and has no shade along the way, so I advise against it; the boat to MGM is air-conditioned.