I think you've gotten some good feedback. However, I would look at it from some different angles, as well.
(1) How soon, or how often will you be going to Hawaii? And is it important to you, or not -- that he remember the trip?
The younger they are, the less of the trip they remember. A pool, beach & playing in the sand could be 5 miles away or 5,000 -- it doesn't really matter for them. If they can splash & play, they are happy. When they are constrained & cannot play, they are usually not so happy. Their world is much smaller.
(2) How good of a swimmer is your child now? Can your child snorkle? How does he do swimming & playing in the ocean?
I ask this because a big part of a Hawaiian vacation is hanging out at the resort pools, snorkeling and enjoying the amazing underwater marine life. You will feel less stressed out at the pool if you know your kid is a good swimmer. Pools and little ones can be very stressful, as far as keeping an eye on your kid, making sure they don't run/slip, stay safe, stay away from the deep end, avoid dangers, don't harm anyone or anything else, etc... there's a lot that can happen or go wrong. And it can definitely affect your ability to relax and enjoy your vacation with your spouse.
When my daughter was 3, we took a 2-week family trip to the East Coast (1 week Disneyworld, followed by 1 week on Paradise Island/Nassau). We had a nice time, but it was a long trip & flight with a little one. They still need their midday naps, and find a lot of things scarry. She didn't remember much of that trip, at all. The lazy-river at the Atlantis she just remembers as "the pool". Because of the huge aquarium & shark tanks, she was always worried that somehow the sharks would find their way into the pool. We must have told her 100 times that they wouldn't. The pool slide was really too high, and too much for her, even when going down with an adult.
After that experience, we waited until my daughter turned 5 (the summer before kindergarten) before taking on Hawaii. By then, she had a lot of experience swimming in resort pools, and was a decent underwater swimmer. That spring before we left we signed her up for group swim lessons at the Y, hoping to turn her into an excellent swimmer. We ended up having to do private lessons with one of the swim instructors, because during the group lessons the kids spent most of the class time hanging onto the side of the pool waiting for their turn. Then, just before the trip, we worked with her on using a snorkel.
Right at her 5th birthday, we did the 2 week Hawaii trip & it was fantastic. Probably one of the best vacations we had ever had. On Kauai, she enjoyed all the resort activities at Ka'Eo'Kai -- learned the hukilau & performed it at the resort show, swam at the little pool every day, snorkeled at the little cove in Princeville every day, did all the arts & crafts activities, took pictures at every waterfall, kayaked twice, watched sunsets everyday up on her dad's shoulders, hiked up & down several scenic canyons, chased wild chickens whenever she saw them. On Maui, I remember much more driving --- the road to Hana (stopping at every waterfall), the road up to Haleakala volcano at to watch the sunrise, more snorkelling (but not so much in the protected coves). Maui was a little bit more stressful than Kauai - big resort pools, lots more driving (not good with little ones), more open ocean beaches, rather than little coves, and unfortunately not many resort activities. We also spent a couple of days on Oahu, and enjoyed snorkelling in Hanauma Bay (shallow water, good clarity, lots of fish). It's also a great snorkeling spot for little ones. We also went to Waikiki Beach and Diamond Head, and it was okay -- but definitely Kauai was the most enjoyable part of the vacation for her.
I post this long story just to point out the huge differences in travelling to distant places with a 3-year old vs. a 5-year old. The difference in their ability to enjoy it, understand it, and delight in the differences (vs. home) -- is enormous. Plus, I would advise you to pick your islands & resorts carefully, to make sure it is all very child friendly. Shopping and lots of driving are not usually interesting to children, so you may want to think ahead about what types of things you, your spouse, and your child would enjoy doing together.
--- Rene