If you walk from Boardwalk to the international gateway to Epcot, you will arrive in about 10-15 minutes. If you take the boat, it will be about the same amount of time assuming you don't have to wait for the boat. We always figure it's easier and faster to walk. In fact, we often get off the boat at Beach Club when coming from DHS, and then walk from Y&B to Epcot instead of getting off at BW or waiting until the Epcot stop. We always get there FAR ahead of the boat.
If you assume the boat starts at DHS, it will stop at Swalphin first, Y&B, BW, and then Epcot. In fact, we often get off at the Swalphin and walk from there and even then always get to Epcot before the boat.