We've done western twice and did snorkeling adventures in GC both times and both times they were amazing. Stingray City Reef Sail & Snorkel (G09) & Shipwreck and Reef Snorkeling (G15) were fabulous and reasonably priced. Our DDs were 9 the first time and 10 the second while DS was 11 & 12. Kid friendly, fun, stupendous experience all around. If you can't tell, we all loved the snorkeling in GC.
In Cozumel the first visit, we did the Tulum Ruins (CZ09). We're history & culture buffs and highly recommend it. For a while all cruise lines suspended excursions to the mainland due to safety issues, and I'm unsure if those have resumed. If so, Tulum was a big hit for mom, dad & kiddos. The second time we hit the Punta Lagosta shopping area (fun) then dropped the kids back on the ship while DW & I explored Cozumel en route to the Cozumel museum. Got a guided tour by Oscar and had a very enjoyable experience. Recommended.
Jamaica was a severe disappointment. We did Chukka Taste of Jamaica at Good Hope Estate (FA28) and were let down on all fronts. The house was poorly maintained, the docent, while enthusiastic and friendly, wasn't armed with the history of the plantation beyond Wikipedia, the food was late and there were seemingly intentional delays (45+ minutes) where we could buy stuff at gift shops. Hmmm. Yeah, not recommended.
Our second Western we went to Costa Maya. The area directly off the piers was a touristy area, but DW got some good tequila for a decent price. Beyond that, not so much. A friend of mine from work visited there with his fiancée and they did a backcountry ATV type tour in the jungle and they had a good time. If we ever go back, I'd try that.
Love the Western cruise, enjoy your trip!!