Cruised the Grand to Hawaii in March this year. I recall seeing a taxi in Kauai, but not other ports. But then, I wasn't looking. The port shuttles were running, Walmart and Hilo Hatties. Taking those, we enjoyed walking to places nearby, but I understand this is not what you want.
Related to what you ask, we planned to rent scooters on Maui, (Kahului) knowing about the rental car shortage. When we got there, the scooter place was not to be found. The usual place was also gone on Kauai. So, if you do rent, don't plop down a deposit online. They may not be there. Scooters, there's a thought.
You didn't say when you plan to go, but as of March, Hawaiian tourism had not recovered and prices on everything were very high, compared to Los Angeles. We were so happy to get back to cruising, but it's going to be awhile for things to return to pre-Covid.