I forgot to reply to your other queries.
dive gear to use this time of year: I used a 5mm and a hood and gloves this time of year as the water will be in the 70s and I was doing a lot of diving. It will depend on your comfort level and how much diving you plan to do.
Do I need to bother w/a rental car or can I shore dive from the resort? Are dive shops willing to pick up a solo diver? YES! you need a rental car to get around the island. I don’t know what the resort offers as to shore diving, but no dive shops in Maui pick up divers, whether group or solo.
Does anyone have any experience w/cruising the islands? Can I pick up the cruise in Maui vs flying to Oahu? Should I take the dive excursion thru the cruise ship (pricey) or make my own diving arrangements (potential hazzle)? I don’t have any experience with cruising the islands, but I believe cruises come in and go out of Kahalui (same area as airport). I’ve never taken a cruise, but people I know who have (taken caribbean cruises), they’ve told me that if you take an excursion through the cruise ship, the ship is guaranteed to wait for you to get back before departing. Whereas, if you make your own diving arrangements, if you get back past departure time, you’ve missed the boat and have to make your own arrangements to catch up with it, at your own cost.
I hope all this helps. Have fun in Hawaii. Aloha.