I think you'll find that the walking times vary quite a bit, based of course on how fast you walk. 15 minutes is about the shortest I've ever seen anyone say it takes, and I've seen some say it takes 30+ minutes.
If speed is your priority, and you can walk pretty fast, walking is almost certainly the fastest way. Even if you happened upon a boat as it was preparing to leave Epcot, you'd probably still beat it, but again it depends on your pace.
Depending on the weather and how much of a hurry you are really in, the boats can be a more relaxing way to make the trip though.