I don't know about Jamaica. The Cayman Islands are fairly conservative. In Mexico (Cozumel) I have seen and heard of women on the beach topless. However, I don't know of any beaches that are known to be topless.
However, I do know of two beaches that are well known as clothing optional / nude beaches.
1) Orient beach on St. Martin - Eastern Caribbean
2) Haulover Beach, Florida (yes - Florida, USA) - It is on AIA just south of 826 and north of 922. This is either north Miami or just north of Miami)
Have fun!