Thailand is wonderful in February and March, in my opinion. Though I wasn't there for a diving trip, we did some snorkeling in Koh Tao and Phi Phi Island and had a great time. A little cool in the water but not horrible. If you're looking for nightlife Koh Tao doesn't have a lot to offer but it's fun. I suspect there is definitely better diving and better "night life" if you're looking for the "ville" type scene with lots of bar girls and the like, but we had a great time climbing, snorkeling, camping and relaxing.