Phuket is actually separated from the mainland by a narrow channel that is spanned by a causeway. Most the visitors arrive by plane or car, but there are regular ferries linking southern Phuket to Phi Phi Don island, Krabi (Ao Nang Beach) and the Yao islands in Pha Nga bay.
can get crowded in the high season, they aren’t terribly modern nor comfortable but you can sit on the top deck and tan. Be warned about the hazards of over-crowded boats, but accidents are rare. There are also private speed boat transfers for hire at premium rates.