And if you are a fast walker, you can even beat the boat. Although, we've never tried it anytime except December... might not want to race it if you'll be there in the hot months.
I could be wrong, but seems like the walk is 1.2 miles or there abouts... anybody remember?
As far as the bus vs. boat goes... what part of Epcot are you wanting to see? The boat takes you to the International Gateway (between Great Britain and France on the World Showcase) and the bus would take you to the main entrance (by Spaceship Earth).