There are some variables here...if you are leaving MGM, and there is either a boat at the dock getting ready to load, or there is a boat on it's way into the dock, then you may want to take the boat. Once the boat leaves the dock, it takes about 30 mins to get to Epcot..sometimes a bit less. But, if the boat is just pulling away or there is no boat in sight headed into the dock then you are much better off walking. It takes about 25 mins to walk from MGM to Epcot...and that is at a leisurely pace. I make it in about 18 mins but I'm a fast walker.
Once you get to Boardwalk (it will be on your right as you walk on the sidewalk) you can cut through past the pool area. Then keep going to the underpass, just past the pool, and take a left as you come out. You will be at the Village Green. You will be almost to the boardwalk itself at this point..take right when you get to it and keep on going to Epcot. If you stay on the sidewalk, rather than cutting through, you will come out in between Atlantic Dance Club and Jellyrolls, which puts you on the boardwalk but much further away from Epcot than if you cut through by the pool.