We did Hawaii aboard the Infinity in April. I rented cars in advance (Alamo - about 6 months in advance - I was anxious!). We never had to call Alamo when the ship docked - their shuttle was waiting for us at each and every port. In fact, they had our rental car waiting when we docked at Hilo - all paperwork was done at the pier then we were on our way to explore.
We were glad that we did it this way.
We ordered literature about the islands well in advance of our cruise so we knew where we wanted to go and how to get there from each port of call. There are lots of good web sites for gathering info on Hawaii.
Enjoy your cruise.