I will be on a 'dam' ship next month to Hawaii. I used Budget on Oahu, Hawaii (Hilo and Kona), Maui and Kauai. I talked to a Budget agent yesterday and was told that there is shuttle transportation from the dock to the airport to pick up the car. I even got the office phone numbers for each Budget on each island. I can call them when we're about ready to dock and have the shuttle waiting for me.
There were at least two places, Honolulu was one, think Hilo was the other, that have rental car curtesy phones at the end of the dock for you to use.
I'd suggest you call Dollar's 800 number and ask if there are shuttles in each port. I wouldn't be surprised if there isn't!