As you see, there are great features to each island. I have lived and dived on Maui for a total of 10 and a half years. I have done 20 or so dives on the Big Iland, Oahu and Kauai. Would have done plenty more if conditions and weather had allowed. I also lived in south east Florida for 5 years. Here is my take solely based on my experience.
Oahu-mostly very busy city or suburban life (very , very bad traffic in urban areas) with the exception of scattered pockets of rural life, mostly northshore. Shore diving is accessible but less accessible and fewer spots than on Hawaii or Maui. More spectacular dives on Oahu are Northshore almost exclusively in the summer and wreck dives (mostly only accessible by boat) which team with as much life as any reef (though they may still not interest you.) If you have experienced any rock fever on St. Croix-missing shopping, chain restaurants, big name entertainment, etc you will feel this less on Oahu.
Kauai- quietest main island. Most northerly which puts it a bit further from equator and can result in a couple of degrees cooler water on average throughout the year. Therefore, has smaller coral reefs that are less developed than the more southerly islands. Still has great accessible shore diving but due to the shape of the island being somewhat round the wave action wraps around moreso and can effect the quality of shore diving in more locations. Northshore has interesting underwater terrain with many turtles and sharks but less developed corals. South shore hase more developed coral and more sheltered sandy beaches. Unfortunately, from the most beautiful, huge, sandy beach on the island -barking sands (Polihale) there is no coral reef and often screaming current and shore break. Whole island is a beautiful place to hike. Peaceful but rainier than other islands. That's what makes it the garden isle and keeps more of it green. Basic shopping, amenities and conventional entertainment are much more scarce but diving, freediving, kayaking, hiking, and learning tradition lifestyle abound here. It should be noted that the trip to Lehua from Kauai is very weather dependent and a touch ride most of the year but worth it if you can make it.
Maui- The reason that I have found Maui to be the best fit for my diving partner and I is that there is very accessible shore diving on all sides of the island once you learn to understand the weather patterns and sea patterns (rarely Northshore diving in winter). There is almost always a shoreline with excellent diving and they are all within reasonable driving distance. Even the most distant coast from our location is only a 2 1/2 hour drive. Now, due to the remoteness and need to go to altitude to reach the most distant coast from us (by the most reasonable path) we only dive there when we can camp and have a safe interval after diving before going back to altitude. There are more than 30 dive locations from shore within 45 minutes of our house. Altitude inbetween sites sometimes comes into play on both Maui and the Big Island (Hawaii). Most of our shore dives are from 15 to 45 feet of water which makes for long, fun dives much like the BHB (Blue Heron Bridge for non-floridians). My friends from there love to do 2 hour dives taking photos of critters that believe it or not you can't even encounter at BHB. We also have great boat diving, kayak diving, and scooter diving.Don't discount the wrecks they are where many of our most elusive critters like to hide. Mala Wharf, an old collapsed pier, will remind you of BHB with a little bit trickier walk in entry. We frequently see larger critters like sharks, rays, turtles galore and the occassional monk seal and can always find inumerable nudis and tropicals. Even here we occassionally have seahorses. Island amenities are mostly equal to any mainland area but you can be in almost any climate that you choose. Keep in mind, living upcountry on the mountainside would make diving tricky. There is plenty of hiking in rainforest, bamboo forest, eculyptis forest, native tree forest, ancient trails, and beach walks. Shopping is easy with local grocers for some of the best local produce and meats, farmer's markets, Safeway, Costco, K-mart, Walmart, Ace, lowes and Home Depot. Entertainment is both local and international with the Maui Arts and Cultural Center, Film Festical, Jazz Festival, Hawaiian, Chinese, Japanese, Philipino, Korean, and other South Pacific Islander culterual events year round. Healthcare is much better than many want to admit and this has been significantly important unfortunately. BTW, shore visibility usually averages 35 to 65 feet. Note: If you live Kihei side where you have almost constant sunshine and mild climate you will have sugar cane smoke some seasons.
Hawaii, the Big Island- Another great place to live. There are plenty of places to shore dive. The most consistent are usually on the Kona side and south from there. The shore lines as mentioned are more rocky with fewer sandy beach entries but this and the quick drop off in many areas makes for good clear water. Shore dive visibility averages 50 to 70 foot viz with frequent viz in the 80 to 100 foot range at the better spots. Coral is healthy and abundant. Larger life such as dolphins, rays, pilot whales, and sharks are sited a bit more than other islands. You have great boat, kayak, scooter and walk in diving. Drives to dive sites are a bit longer if you wnat variety. from furtherst point to furthest pint on the island I believe is about 5 or more hours and would involve going to altitude. Kona side is one of the driest sides but does not enjoy as much sun as the drier sides of other islands due to the height of the mountain and the frequent Vog. Even though you are dry and warm, you will often look up to gray skies and have thich, humid, voggy air on Kona side. Hilo side gets more rain though often broken up by sunshine and rainbows. Hilo just has trickier and less diving. Amenities as far as shopping are pretty good though you will use more gas to get to some options but well known entertainment is fewer and farther between. Local stle events and cultural activities abound. Lots of open spaces to explore if you have the budget for the gasoline. Most folks pick a side of the island though and pretty much saty to that side.
Twenty five percent or so of our marine life is endemic. We have nearly twice as many tropical fish species as the caribbean, more than twice as many varieties of eels and lobster. We have far fewer and smaller sponges and soft corals due to our remoteness, the wave action that we see and the cooler temperatures. Other than in the shallows on a sandy beach the water temp is rarely above 80 but it also is usually above 74 almost year round.
Good luck with your choice. I wholeheartedly agree that you need an exploraory vacation here first. At lesat a month would be best if at all possible.
