From Portofino Bay, I would take the boat...it's a hike. From HRH, just walk to either park. From RPR, it's a toss up. If there's a wait for the boat, it is usually faster to walk, especially if you're heading to IoA since the walkway drops you right up from the entrance to the park.
Because the boats arrive on the opposite side of the water from the parks, you need to add a bit of time to get from the boats to the parks. US is quicker than IoA.
And during the less busy times of day, they pretty much run one boat, back and forth, from each resort. So, if you just miss the boat, it could be 30 mins before it returns.