I reserved a separate car on each island, of course, kuai, oahu, maui, molokai, and big island. I reserved ahead of time. It worked like clockwork. BUT, I flew to each island. It took me an hour or two for each one to get in the car, well one or two were quicker.
You will really be pressing it on your cruise.
If I were you I would enjoy a slow paced visit near the port, or take an excursion. And I would fly back later and take your time seeing the sights. To really enjoy a hawaii rental car you need a full day or two
Well, just remember, could be time consuming checking in/out. Not sure about cancellation policy. You can use Expedia to see all the rates and then maybe call up the individucal rental center and talk with them. But a person can promise speedy delivery all day, but still not deliver. The time I was in Maui, a tragic crash drastically limited traffic for several hours.