February to May is probably the best time to dive the Similan Islands and other dive sites in the Phuket area. Seas get calm (the east winds die down), weather is hot, big fish start to arrive (no promises), its not peak season so flights and hotels are cheaper than Dec/Jan.
You have a huge variety of options on liveaboards, plus daytrips (a day trip is not really worth it in my opinion, too far for just 2 dives).
Vis should be just fine.
Also, while you are in the area, you really should try to dive at Hin Daeng and Hin Muang.