You can take the Friendship boat from the dock at the front of DHS -- it makes stops at the Swan/Dolphin, Boardwalk, and Epcot. It is a nice ride, though not exactly speedy. It could take anywhere from 20-45 minutes, depending on time of day.
The other option is to walk -- there is a pathway along the water, I believe it would take about 15 minutes if you were walking at a pretty good pace (I haven't done it myself). I do know that most (if not all) of the path is uncovered, so that 15 minutes may feel a lot longer under the beating sun. Probably a better bet in the morning or evening.