Western Carribean excirsions with a 6 and 3 year old

[Ljaecruise]

We will be going on our first DC this January on the Fantasy. I have two kids aged 6 and 3. Any must do excursions on the western itinerary? Has anyone done Sting Ray city in Grand Cayman with a younger child? I know it is for age 5 and up but my 6 year old is not a great swimmer. How high is the water? What about in Cozumel or Costa Maya? TIA!!!