January in the Bahamas can be brisk.  54 degrees on the water with a 20 knot wind in your face can feel rather chilly.  My favorite and most used clothing items are as follows.  On a cold or cool sunny day, I wear as a first layer, nylon running shorts and a tankini (bathing suit) top. Over that, I wear some long thin - cotton workout pants (thinner than sweat pants) with a pull - on waist, and on top, a thin polar fleece zip jacket with a hood (thanks, Barb).  The hood and jacket act as protection against the wind and the polar fleece feels great against my skin.  As the day heats up, I can easily remove the outer layers in whatever combination meets my needs.  At night, it is easy to rinse out the running shorts and swim top and hang them up so that they are dry by morning.  Since the running shorts have little panties in them, they are really comfortable, they dry easily if you get splashed, and it cuts down on the need to wash another article of clothing (like underpants).  I wish I hadn"t turned down those sticky socks my mom wanted me to take.  You know, that socks that have some type of sticky appliqus on the bottom so you won"t slip.  Writing at night, my feet get cold,  so that's an item you may want to consider bringing.