Last spring I cruised to Hawaii from Ensenada. We were in Ensenada long enough to get off the bus and on the ship, so I am not much help there.
As far as the cars go, they don't or won't bring the cars to you, but they have shuttle busses that will come to the cruise ship/terminal and pick you up taking you to either the airport terminal or local rental car place.
My experience follows:
Lihue= Hertz picked up at ship by Hertz shuttle and got my car at the Marriott. Returned the car to the airport and was shuttled back to the ship. Very efficent.
Maui=Hertz picked up at the ship in Lahaina via shuttle and rented from Hertz office in Kaanapali. Returned the car there and was shuttled back to the ship. Very small office and it was a bit time consuming.
Hilo=Hertz pickup at ship and shuttle to Hilo airport where I rented the car for the day. Returned to airport and shuttled back to ship. Don't miss Volcano's National Park here, it's worth the trip!
Kona=Same thing Hertz pickup and shuttle to airport. Kona airport is about 15 minutes from where you are tendered in so, I had to come back and then we were on our way as they would not allow but one person in the party on the shuttle.
Overall the car rental agencies are very familiar with the cruise ships, they rent all the time to them. Make sure you call in advance and not just that day because in places like Hawaii where there is a lot of tourism you might get left out if you don't reserve early.