Four options:
1. Take bus, takes 10 to 12 minutes once the bus leaves but you may have to wait for a bus.
2. Take boat all the way from MGM to Epcot, takes about 30 minutes due to stops at Epcot resorts (Swan/Dolphin, YC/BC, and BW) before reaching Epcot.
3. Take boat to Swan/Dolphin and walk over the bridge there to BW and down the boardwalk to lane leading to Epcot. That total trip takes about 18 minutes as the longest leg of the boat trip to Epcot is between MGM and S/D and walk to Epcot from there is fairly short.
4. Walk entire way -- about 25 minutes.