The boats dock near the International Gateway (we call it Epcot's "Back Entrance").
This entrance/exit is centered between the "lands" of the United Kingdom and France (if you walk down the divergent sidewalk toward some small shops, you'll find the gates.)
Assuming a "normal walking pace", walking can actually be a bit faster than taking the boats (as there are 3 full stops for load and unload during each trip, in either direction, between Epcot and MGM Studios).
The actual time to ride the boat is generally closer to 30 minutes, but, that can vary a bit.
Walking takes about 15 minutes (if you don't stop at Boardwalk to sight-see or sip a cappuccino, but who can resist?)
OTOH, if the boats are not too full (like at closing, when there can be quite a line) they are a very pleasant ride.