Ok...gonna wade in here. I used to coach swimming and was a beach lifeguard, the beach isn't like a pool.
Yes, it's easier to float in salt water, but LEARNING to float in water that might be 10+ feet deep isnt' something most novices can do.
In Cayman, do the Stingray City tour, as other's have said you can easily stand, But understand that the ocean is predictable. A life vest is there to help you, not save you.
You have time, i would suggest taking some swimming lessons. The red cross has wonderful classes for adult swimmers, if that is not possible, I wouldn't advise going into water in the Ocean that you can not stand in, it's too unpredictable.