Last week:
Key West - did not test
Grand Cayman - the most pleasant. Slightly cool but definitely swimmable
Cozumel - I thought a little bit cooler than GC but again doable. We did get a little chilly towards the end of the 30 minute Dolphin Encounter at Chankanab
Castaway - The coolest of them all. A little bit cold when you first got in for a snorkel, but you got used to it (at least I did). However, my 8 year old got very cold on the snorkel and after about half way we turned back so she could get out and warm up. She did not get cold snorkeling in GC or Coz.
Standing in waist deep and playing she didn't seem to have a problem though.