On Hawaii here and just to add to the pile, I do see people including some locals on occasion diving w/o a suit at all. Most common those in a shorty or a skin (when IÃ×e been out on a boat) claim they came up because were cold.
Upon arriving I thought wearing a wetsuit, even a shorty here was insane. Then sorta fell for a sale (+ recommendation) and got a 3ml shorty. Never went back to bare after the first time snorkeling, big difference in comfort.
Then in BOW class in a pool, cold and chattering and got a 3 ml full suit that has served me well. Now I suspect IÃÎ more acclimated to Hawaii and diving (less effort expended) so my 5ml is the main used. It can be overkill but easily flushed at the neck so no biggie. 4 dives and I am adding a 3ml core warmer too.
Boat dives on the Big Island West side are rather shallow easy sloping reef and tide is no more than a couple of feet. Point is not a lot of exertion, no swimming out and in or direct dropping down deep (say onto a wreck off Ohau.) Water temp does vary between islands.
75degrees is freezing for me, 78 is cold and at 80 I am in my 3ml. I am not recalling temps above 82 by my computer. Most commonly the temp is little deference surface to 100' 1 -2 degrees. Un-diveable conditions are very rare.
So being a furnace you might consider not purchasing at least for Hawaii. Doing some dives and see how it goes. There are plenty of rentals unless you are some odd size. And perhaps just pick up a core warmer or shorty here will be enough and you might find as I did, you didnÃÕ realize you got cold, until you felt what staying warm was like.
I did not realize how nice any head was until not having one onboard and recommend
Jack's Diving Locker for a nice head
J. A larger boat than some but I do not find myself having the urge to Mooo. Also has hot water rinse off the back and I have always found the crew delightful, one way or another and they are not rigid about 1 hr dive time. If you are limited time on island I recommend booking early as they are generally busy if you choose them.