In general Phuket has nice diving, with the Similans and Hin Daeng/Muang being the highlights. But also the other daytrips are worthwhile, with nice macro life (sea horses, ghost pipefish etc, bigger life like jacks, sharks, tuna, turtles and all the normal reeffish. And there even is a wreck, the King Cruiser. As a wreck I don't find it that interesting, but it really is a lovely divesite with lots of lionfish, scorpionfish, hunting jacks, morays, sharks.
Regarding the weather: fortunately the viz is not influenced very much by rains, but the diving is better of course with sunshine. And it can be rainy and it can be sunny, there are no trustworthy weather forecasts. But whole year round diving is good.