Does it still feel like Mexico? If it is just a fancy dressed up folk place now, I might not enjoy it. I sure am missing the chance to go to Scuba Club.
Been going there since 1997 and there's really nothing fancy about it. I'll take one or two collar shirts (short sleeve fishing shirts) and one or two decent khaki-type shorts, but really even that level of dress is not needed for most places other than some of the slightly more upscale places like Buccanos At Night, Casa Mission, Azul Madera, Kondessa. There may be more fancy places I'm not aware of, but those are nice enough for my crew.
Basically the rest of the island is T-shirt and bathing suit or gym shorts. Obviously some places don't want you dripping wet, but most places are pretty casual.
As far as feeling like Mexico, the town is more touristy near the water/malecon, but as you walk inward to 30th it gets more of a local feeling, and by 65th you can still find places where a little bit of Spanish helps and you can still have a good meal under $10.
As far as reefs, they say "comparison is the thief of joy" and I think that also applies to comparing the present to the past. Some people say Cozumel reefs have declined over time. I've only been diving since 2018, snorkeled before that, and having gone multiple places in the Caribbean (Bahamas, Bonaire, Belize, Curacao, St. Croix) and elsewhere, Cozumel is still my favorite. I hear people say it doesn't compare to Asia, but it's much closer and still pretty great to me.