For pure diving and overall weather conditions, Bonaire is hard to match in January. As it is in the Southern Caribbean, Bonaire has the longest winter days in the Caribbean and has never known overnight temperatures below the mid-sixties. Besides , it has some of the best shore diving and easiest beginner diving anywhere.
You can expect little or no current with gentle sloping reefs starting about 50 yards off shore in 20' to 30'of gin clear 80 degree warm water with the reefs extending down to about 130'. No dive location in the Caribbean can boast more species sighted than Bonaire.
With American now flying to Bonaire, the air options are much better than ever (we got Miami to Bonaire for $267 each on American this January) Dutch Caribbean Airlines and Air Jamaica also offer specials from Miami.
Cheap accomodations are available although there are no chain hotels on the island (other than the Divi Flamingo). We always stay at a long term apartment (the Yachtsclub) for about $55 a night for a one bedroom apartment. Check out
www.infobonaire.com for more information.
Shore diving is the way to go, most dive operators offer unlimited air and weights for around $100 for a week (available 24 hours a day) and there is an abundance of good operators so it is hard to go wrong. If you would like to do maybe a boat trip or two over to Klein, these are reasonable and, usually fairly short for around $25 each for one tank.
All you need is one truck rental ($200 week)(we got$600 per week for truck/apt for 2 complete this January) to get around town and out to all of the dive sites.
While the nightlife is not quite like Cozumel (no Senor Frogs sping break danceathons), there are two casinos (including the Divi, the world's only barefoot casino) and a few good dance bars including our favorite, Karel's on the pier in downtown Kralendijk.
Finally, you can drink the water with no fear. As it is a part of the Kingdom of the Netherlands, it is a generally clean island with first world infrastruture. While I love Cozumel and like to spend considerable time in Mexico, potable tap water is a nice touch.