Visibility is good in January but for atoll trips and in case you want to do some sunbathing, I would recommend March. The seas are generally calmer with less northern winds from coldfronts so you can get out to and back from the atolls without risking a hernia or dislocation from twelve foot seas and it is usually a bit more predictably sunny. January can be hit and miss. If you hit it right, it can be great but if you hit it wrong, it can be not so great. It is definitely true that proliferation of microrganisms and plankton, tiny jellyfish, etc. is higher the warmer the water gets, but March is just at the end of the cooler water season and is usually very nice. But as the other poster says, even at the height of the warm waters, Belize could still easily have the best diving many people have ever had. And usually when the tradewinds are blowing nicely, a lot of the fresh water runoff along the coasts stays right there along the coasts. The barrier reef is a good ways out from the coast and the atolls can be 25 to 45 miles out from the coast.