We need the context of your question, Skepticphil. What are you looking for? Depths range from zero, on the long sandy shallow water beach, to those at various dive sites (here), to thousands of feet in some of the deepest waters in the Caribbean.
Well it starts from shallow but as you swim out it does get deeper. But there are cordons marking out the safe swimming areas. Go beyond these at your peril.
So are you off to Negril then? I had the time of my life there.
Be sure to go to Ricks Cafe and have a jump off the cliff.
No I don't know how deep the water is there!!
If you do jump, let me know how it went!!!!!