The boat is really nice. BUT. When we were there, going to the boat dock was actually out of our way. We needed to go right to get to the path, but the dock was to the left. Which was fine, unless the boat was JUST leaving...then we didn't want to wait for the boat, and had to retrace our steps.
Also, the boat lets you off at a big dock that is in between IOA and Universal parks. So you have to actually backtrack a bit (going up stairs and then to the bridge) to get to IOA. And you don't backtrack for either park when coming from the RP path (and IOA is closest to it).
Basically, if you are good, quick walkers and no one is going to sweat to death on the way or be miserable, you might consider just walking. If you like being more leisurely about it, take the boat. (there are also shuttles, which I, for some reason, didn't know about, even though it was in the literature AND the guidebook I bought!)