You forgot your Keys and Key Largo in particular!!! We have what you're looking for.
1. warm weather & warm water with good visibility
We've got that and pretty much year round.
2. lots of fish, sea life
Lots of fish, lots of cool stuff like wrecks and artificial reefs. You can even dive and volunteer in a coral nursery through Coral Restoration. We're also the home of REEF.org and the History of Diving Museum.
3. easy to not go deeper than about 80 feet and can spend significant time at 30 to 60 feet where the fish are — i’m more interested in relaxed wildlife-watching than anything deep or adventurous. although i love drifting in cozumel’s currents.
This is where the Keys really, really shines. Most of our diving is in the thirty foot range. We do have some deeper sites, but most of our great diving is shallow.
4. affordable flights from usa (usually to cancun, which offers cheap/easy access to cozumel) -- i think any other destination will require a bit more air fare, so i accept that.
We have both MIA and FLL airports close to us. Few dive destinations are served by two major international air ports.
5. a town i can stay in that is mostly walkable with good food/restaurants
Lots of restaurants but not a lot of tourist traps, pushy sales people, pharmacias or massage parlors.
6. plenty of cheap hotels that are modern & comfortable (i'm not really into resorts or all-inclusives, although i guess i could try it if the diving was really worth it)
We're not the cheapest, but you can find anything from a Mariott to a travel trailer on the bay. Hell, if you have enough money, you can stay in Jule's Undersea Lodge. Yeah... underwater... for reals.
7. mostly boat diving, with a big customer base -- this is pretty important since i can jump aboard a boat with anywhere from two to eight other people plus a divemaster. i prefer this over trying to find a stranger to shore dive with, since i like to just rest or wander around doing my own thing when not diving.
Boat diving is almost all we do! Only two shore dive spots and they are used mostly for training.
8. lots of dive ops, so can find one that you like (safe, longer bottom times, rental equipment, etc.)
We have more dive ops than gas stations and we have plenty of gas stations.
i guess the only thing i dis-like about cozumel is the industry that's developed around the cruise traffic. i'm not bothered by cruise ships so much, but i do get irritated at wildly aggressive salespeople lurching at me 10 times on every block
i understand it, i can work around it, etc., but... that's just my least favorite part of the trip
That's more like Key Weird, er, Key Wasted, er, Key West. No cruise ships in Key Largo or the rest of the Keys other than the one sole exception. That means no pushy merchants trying to separate you from your money, none of the seedier side (unless you go to Miami) and it's clean.
In addition, there is plenty of stuff to do/see close by if the seas turn crappy. Your English will be understood everywhere. You don't even need a passport if you're a US citizen.
