I agree with AlohaJoe - the volume of easy diving makes Maui hard to beat. Find yourself an inexpensive condo in the Kihei area - they're across the street from the beach so cheaper.
The boats all leave from the Kihei ramp and go to Molokini then return to dive along the coast. There's also about a dozen good shore dives in Kihei and Wailea just south of there - some back to back. Almost all easy entry beach dives. #'s 11-19 here are within about 2-3 miles of each other - several run back to back.
Scuba Shore Diving Site Listing for: Maui, Hawaiian Islands
We've dove off a condo property - not a named dive but really decent - it's about a bay over from Wailea Beach. Maui Dreams Dive Co, B&B Scuba, a couple of the Maui Dive Shops are options for tanks - none at a dive site. I know the Wailea Grand has a shop on site - we rented there once to dive Wailea Beach. Also there's at least one other resort with a shop - I think it's on Polo Beach.
You will need a car. The affordable flights mostly go into the airport on the east side and you drive across - takes about 45mins. Or the airport just north of Lahaina but those are often more since they offer easy access to the big resorts in Kaanapali and there's less flights/airlines.There's an local shuttle I used a couple times when others had the car - it picked me up at our condo in Wailea - on the Kihei side and took me into Kihei and once to the Shops at Wailea - I think it was $4? for where I went. You'd still need a car on boat dive days though as the Ramp is a few minutes drive north.
Another option is to stay with 5000 of your closest friends in one of the big condo towers in Kahana/Kaanapali. Depending on where you stay there's a couple average shore dives off the beaches there - Black Rock is pretty good. Old Airport also but it's shallow. Tiny Bubbles is one operator located in one of the resorts there - all they do is shore dive.
Lahaina Harbor is the other dive boat marina - there's 3+ operators there. They almost all go to Lanai, Lahaina Divers also goes to Molokai when the hammerheads are there but it's a rough crossing and blue water dive. The Cathedrals off Lanai are a signature dive.
We stayed up in Napili once - had a dive on-site (Napili Bay) and the next bay over was the Kapalua beach/dive. It's pretty quiet and you would need a car to go for food etc. but the condos are smaller, older and beachfront - more reasonably priced.
It would be impractical to do Kona w/o a car. The boats all leave from Honokohau harbor north of town and there's nothing nearby. The Kona airport is even a few miles north of that. There's a few shore dives down the Kona coast MM4 is named for how many miles it is from town - and it's a closer one. Place of Refuge must be close to 20mins south. The Big Island really is big - lots of driving. We found several really affordable condos off-season around the Magic Sands beach area on vrbo. Waterfront but not beachfront. There are two shore dives there - MM4 and Pahoehoe Beach park. No food nearby - that's a 5min. drive to Kona.
One way to do Kona is find a hotel/condo (Holiday Inn Express is one) within walking distance of Jack's Diving on South Alii Dr. They have a free shuttle to their boat and I've read of someone also getting a ride when $hore diving with them. I believe you could walk to several restaurants nearby - even downtown. It would be a long walk to Kona/Kailua Pier which is a shore dive. You might want to price the King Kamehameha also - it's a Courtyard by Marriott and the Pier fronts it.The pier from Jacks would be a long haul with tanks though and I doubt they'd let them on the Trolley.
At least there, pre-book the car. For some reason the walk up rates are almost double. Don't fly into Hilo to save money - all the diving is on the Kona coast and it's over a 2 hour drive. Plus it's to elevation so no-fly rules apply.
A no car plan could work on Oahu. Find an inexpensive condo/room in one of the Waikiki towers, walk to restaurants, fight the crowds and many of the dive operators pick up at the hotels and take you to their boats nearby. I saw a lot of (I assume) free airport/hotel shuttles. I have not spent a lot of time on Oahu. I have never seen as many people crowded onto a beach as I have in Waikiki. It looked like it would be hard to walk between the towels.
