If you are looking for the beaches where Jamaicans go, you should go to Mammee Bay, three and a half miles from Ocho Rios. Most of the beaches belong to the resorts in the area, among them Sandals Dunn's River, but a few of them are public. Fisherman's Beach is one of the most popular.
Caribbean waters are warm enough to go for a swim all year round. Sometimes waters are so warm, that they are not even refreshing. The problem in beaches like these is the foot protection, without sandals it is not possible to take a dip.