The snorkeling at the Similans is fantastic, and by far the best Thailand has to offer. However, it is quite far for a daytrip from Phuket.
So if you're really looking for a lovely snorkeling experience: go and make the trip, just book a trip that is a dedicated snorkeling trip, where you will be separate from divers. Costs will be around 2900 Baht.
If you're looking for a nice day out with some snorkeling: make a daytrip to Phi Phi or Racha Yai and enjoy the islands and beaches, and do some snorkeling in between.