Absolutely, Hands down, Grand Cayman. Actually, I have been all over the Caribbean and GC has some of the nicest water. Though, the water in the dock area on Cozumel is the most gorgeous color of blue you'll ever see. Make sure and look down at it while you are disembarking.
The water at dock in Cozumel is exactly what I thought when I read the question...it is crystal clear and amazing blue colors depending on depth where you are looking.
We found all the waters on our western to be amazing and clear!
In Playa Del Carmen (Mexico) at the Tulum Ruins - The Carribean Sea was simply amazing to see and swim in. The most bluest, clearest water I've seen in the Carribean thus far!
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