Is this for your novel, or are you actually going diving? If you're going diving, that's great.
If you don't mind a side question, where did the depth "120 feet" come from? Is there something special at a dive site there at 120 feet that you want to see? I don't know if things are different in the Bahamas, but in many places in the world dive operators consider that a deep dive and are hesitant to take a relatively inexperienced diver on a deep dive. Also, keep in mind how many minutes you are going to be able to spend at that depth, especially if you are relatively inexperienced. If there is in fact something to see at 120 feet, you're probably going to take a quick look and then start ascending. If it's something colorful, sure, you may want a light. If it's a wreck you are visiting, then unless it's totally encrusted in pretty coral or anemones or something for which you might want a light, there should be plenty of ambient light to see a wreck. As for temperature, even if it is a couple of degrees cooler at 120 than at the surface, you may be spending just a few minutes at that depth and spending the majority of your dive at something like 60 feet or shallower. I am of course just speculating, as I have no idea what your plans may be.
As for wetsuit thickness, I recommend the same as others have said: at least a 3 mm if not a 5 mm. People generally get progressively more chilled with each dive in a day of multiple dives. For me, multiple dives per day at 77 F would mean 5 mm.