Riptides tend to be very localized, sporadic and generally occur on beaches with a gradual sandy slope. I've not seen a single one in Phuket. The water may be choppy, however, and you may not find that to your liking. You may also find some big waves at some beaches. The more sheltered beach areas (like Patong) are pretty good.

I can't recommend a specific snorkeling trip, but you'd do pretty well going out to Phi Phi. If one side of the island is rough, the other side is usually OK. If you go to Phi Phi, make it worth your while and stay one or two nights. Don't try to make a day trip of it, it's just too rushed and there's plenty to see at Phi Phi.