A tough call.
To be upfront, I have not personally been to Belize but I planned a dive trip with my buddy's but had to cancel at the last minute (they went) Also a good friend of mine who is a diver lives there permanently.
I've been to every island in HI - some more than once. Dove every island but Oahu..
So taking cues from your post
Belize diving is decent or better from Ambergris Key. It's also possible to daytrip to the Blue Hole from there. The Atoll resorts are closer to the better diving but being atolls there's no other activities excpt snorkeling or the beach.Also most only exchange guests once or twice a week.
We planed a trip to Placencia in Southern Belize. The ,majority of the dives are as much as 60-90 minutes by boat, One reason we went is in late spring/early summer you can Dive with Whalesharks - about tghe only place in the Anericas you can reliably do that. It;s also the starting point for hiking, Mayan Ruins tours, cave tubing and both the Howler Monkey and Jaguar Preserves. In fact Splash Dive has a sister co that does all that. Check out the Hamanasi website they do most of them.
Lastly Southwest flies there direct from their hubs, closest to you is probably Denver. From Belize City, Troic or MayanAair do the regional flights. My friend that lives there only goes thru Belize City when he has to.
Nexy post: Hawaii
Actually by far and away, the majority of dives from Ambergris Caye and Caye Caulker, Belize are only 5-15 minutes from shore.
If the divers were to choose to stay on the mainland, then Belize City is not the place to stay. Those catching a liveaboard will usually do an overnight there though before catching the liveaboard boat.
Down near Hopkins, there's Hamanasi Dive Resort or down to Placencia, there are a number of nice places to stay and at Placencia, where I recommend diving with Splash Divers.
But it's true that the dive sites from Hopkins area are around 45 minutes out and Placencia, at least the same amount of time.
An ideal itinerary for Belize is to go inland, near San Ignacio and enjoy the jungle, do the ATM cave and check out some Mayan ruins etc., etc., then head back to the seashore for diving portion of the holiday.
There's lots of ways to do these things and lots of price points.
To spend a week in Belize at a price point of 4500-8500, you'll have the very finest of accommodation.
But you can still be quite comfortable beachfront for $1500/week on Ambergris Caye. Just for example, checkout Mata Rocks Resort.
Dives are around $80 for a two tank and there are a number of fine dive operators on Ambergris.