Are you talking about hopping between US and IOA? If so, it's about a five minute walk - the Hard Rock Cafe sits between the two parks. Very easy to do - one day, we went from US to IOA back to US! Just make sure you get your hand stamped when you leave each park.
We stayed at HRH, and it was about a 5 minute walk to US, 10 (or less) to IOA. We never took the bus, and only took the boat once when we went to dinner at Margaritaville at Citywalk. (The boat lets you off on the Citywalk side of the river, not the park side.)