All the hostels are in Wailuku. Except the one in Lahaina (Last Resort) which is still a long drive to Kihei. One thing about it is that you could stay/eat/dive in Lahaina with Lahaina Divers - it's all within walking distance. The trade-off is they get $350 for Referral dives but they're all done off their boat in the afternoons. And there's 1/2 dozen nearby shore dives if you find a buddy. Maui Dive Shop(s) rent tanks and sell fill cards.
i just heard Maui is wildest than other islands and i'm not a big fan of big cities
Probably don't want Oahu then. They have real traffic jams there daily. And the shore dive sites are spread out all over the island - many of the dive operators are near Honolulu/Waikiki since that's where most of their business is fron. Check with a couple though as they're used to picking divers up so they may for classes/referral dives also - IDK. No idea about hostels there - I've only spent one day there ever.
If you haven't noticed, flights to Honolulu run about $200-250 less than direct to Maui also.
Wildest is probably Kauai. The hostels are in Kapaa - north of Lihue Airport. Seasport Divers has a shop in Kapaa about 2 blocks south of them. They get $275 for referrals. It's also a populated enough area to have an assortment of markets, restaurants etc. I dive with their other location in Poipu - the 2nd best shore dive is at Koloa Landing about a 5min. drive away. They don't keep a boat in Kapaa and do teach at Koloa Landing - I've seen their van there. I don't know if that means you have to get there yourself (20 minute drive) or if they have somewhere local. The nearest place I know of - Ahukini Landing - is now closed to divers.
Kauai will be a more expensive flight than even Maui though. But it's much "wilder". Many places you're still the only one on the road except for around the port/airport or popular beach areas.
Another consideration could be Kona. The hostel there - Patey's - is in town. Drove right by it once downtown. (no I didn't remember the name, just the location) Check with Jack's Diving Locker - they're in town on Alii Drive - you could walk there. The large Kona food markets would be 2-3 blocks from Patey's to the north. Kona Airport is about a $40 cab ride though. There's a shuttle that runs thru town then down Alii Drive to Keahou 5 miles away. Jack's probably teaches at Kona/Kailua Pier - it's a good dive plus walking distance from their shop. Or there's a site a little north of town. If they teach at one of the south sites, the shuttle could be an option - it goes right by MM#4 and Pahoehoe Beach Park - both are dive sites.
From Phoenix either Kona or Maui are direct and price around the same. But we have the USAir hub here so they compete with the LAX-based Hawaii carriers. You might look at Southwest or Spirit then changing carriers here if that's any cheaper? Although with the USAir/AA merger I'm not sure of that either - haven't been to Hawaii since then - it's possible everything routes thru LAX also now.
hth, was bored this morning...