I've been diving since the 1980s, and I've found the differences in the quality of the reefs to be stunning.
During a recent return to Cozumel I was eagerly anticipating a few dives on sites that I had logged as outstanding, especially the deep Palancar reefs which were teeming with fish life on my first trip in 2005 [sorry wrote 2010 here originally- my bad]. I requested we do this reef on our first dive in 4/17. This time around, the Palancar deep reefs were desolate wastelands almost devoid of marine life. There were a few dives on that Coz trip that were still really nice and approached the quality of the original trip in 2005 but the differences were clear.
During a recent trip to South Florida I didn't find one reef dive to be worth repeating, it was actually depressing to drift along reefs devoid of coral and fish, although the wrecks were great and still had high concentrations of marine life and the rock piles in between the wrecks of West Palm Beach were loaded with fish of all sizes including Goliath groupers and sharks.
Indeed, Seadwellers of Key Largo have a blog on their site with an article that says 90% of the reefs in the Florida Keys are destroyed.
Trips during the past 5 years to Bonaire, Roatan, Grand Cayman, and the Bahamas will not be repeated, it's simply not worth the time, the expense and the effort to dive relatively barren reefs. Even Cozumel is somewhat questionable at this point.
This week I've read about mass destruction of reefs, the floating plastic garbage pile in the Pacific that is 3x the size of France, and saw pictures of the island of Midway covered with plastic garbage. Aside from the wrecks the number of places to enjoy a quality dive in the US and Caribbean are becoming few and far between.