Yes there is a service, it costs about 500baht from memory. Do a google for Phuket to Ao Nang.
Takes about 2 hrs, we caught the ferry from Ao Nang to Phuket and changed boats not too long into the journey at sea so watch out for this as communication is bad. If you stay on you'll end up at koh lanta or somewhere like that. about 7 years ago
Try this page from our friends at Travel Fish: http://www.travelfish.org/region/thailand/southern_thailand about 7 years ago