The boats are the preferred method IMO, but we like to take our time, if you're in a hurry, then walking will get you to IOA quicker, especially if you are hoofing it. Really depends on how much time you need to save (i.e. whether or not you're rushing for early entry or whatever). I never timed the walk per se, but its about 7-8 minutes from RP to IOA. You come out of the trail from RP basically right at the IOA entrance, whereas the boats take you to the main dock at City Walk and past this spot.