There are several dive trips going to the Similans from Phuket, Khao Lak and Ranong. Both day trips and live aboard cruises are available. Dedicated cruises usually start at 4 days and 4 nights 4d/4n in length. Many shorter trips are available, often with operators who have a large vessel on the islands over summer and who do regular transfers using smaller speed boats.
Note that there is a national park entry fee of 400 baht per traveller and a further daily usage fee of 200 baht per diver per day. Most boats do not include this in their quoted price. It is paid to your dive operator before the trip.
From Phuket
Some diving liveaboard trips depart from Chalong or Patong. Most departures actually leave from Tap Lamu near Khao Lak but focus on the Phuket area, with offices in Phuket and pickups from major beaches