Generally speaking...the boats run the course of WS (Intl Gateway) BW, YC/BV, S/D then MGM. Then it is reversed going back. You are left at a dock that is off and over to the side of the main entrance to MGM.
If you walk, it is about 20-25 minutes depending on your pace. My family usually does it in just under 20 minutes..maybe 17-18. But we have long legs, even the 12 y/o!!!
So, should you take the boat or walk....well, if there is a boat loading the the Gateway and you can get on it, then do that. If there isn't a boat in sight, just walk to the resorts and see if there are any there. Only problem with doing this is, unless you are sure where the boat is coming from, and then going to..you could head to a boat that is going to where you just came from!! If I miss the boat at Epcot, I just walk to MGM. Unless it's very hot/humid. A lot of the walk, after BW, is out in the open and can get pretty steamy. The pathway is very nice, and well lit. It runs along the parking lot area and on into MGM (The Studios).
It's always one of those 'make up your mind as you go' things for us.