I did the walk when I wanted to get one of the little ones to take a nap. Pushing the stroller along the river is very quiet and relaxing, so they fell asleep rather easily.
So I'd say if you have a small child due for a nap, walk. If you want to save some energy for the parks and don't have a compelling reason to walk, take the boat. Probably about the same time-wise with stops, etc...