The Similans are surrounded by open ocean some 70 km due west of Khao Lak beach and are reached only by tour company operated speedboats. While many of these disembark from Thap Lamu pier, which is the official Similans pier near national park headquarters in Khao Lak, many tour companies have their own piers in different locations, though none are too far from Thap Lamu.
Many tour operator offices are located around Thap Lamu pier and throughout the Khao Lak area, and it's also possible to book through resorts, guesthouses, and travel companies (they will receive a commission for this service but the price remains the same for travellers either way as the tour companies typically pocket the commission for themselves when you book direct through them).
Day tours cost between 1,800 baht on the low end up to 3,500 baht, and it sometimes pays to shop around and fork out a little extra for a better experience. These rates should include lunch and snorkelling masks. If you need fins, you should be able to rent these from the tour company for an extra charge, and if staying overnight you can arrange to rent snorkelling equipment (we were charged 100 baht plus 1,000 baht deposit).
Also note that it can be worthwhile to choose the tour company you want first, and then ask for rates from a few different travel companies as some offer cheaper prices than others (and presumably take less of a commission). With this being the case, travel companies often quote cheaper rates than the actual tour companies. We went with Medsye Tour, a friendly operator based at Thap Lamu pier that had been recommended to us by a few travellers.
Many tours also depart from Phuket but it’s more expensive and the 3-4 hours of added transport time in the minibus shuttle makes for a very long day. If you’re staying on the south of the island, expect a departure time of about 06:00 and a return at 19:00-20:00. Especially given the reckless, speed-freak attitude of some drivers, we recommend booking from Khao Lak if possible to keep the minivan travel time to a minimum.
Accommodation zones, beaches, and viewpoints on islands # 4 and # 8 are connected by sandy foot paths with well marked signs, but don't forget to bring a torch if staying overnight.
If staying on the islands, the only ways to get from island to island are to take part in daily trips aboard a flat bottomed boat operated by the national park, or charter a private longtail boat. However, longtail charters must be arranged beforehand at national park headquarters at Thap Lamu pier, and there is no guarantee there will be one available. Unlike at Ko Surin, there are few longtails operating around the Similans, with most of the boat traffic being either tour speedboats or diving vessels.
National park boats run twice a day from island # 8 to # 4, leaving at 8:00 and 13:00, and returning at 10:00 and 15:00. The cost of a one-way trip is 200B. Snorkelling excursions on national park boats depart only from island # 4 and depart twice per day. The first trip is from 11:00 to 13:00 and the second from 13:30 to 15:30. These typically visit some of the more obvious and easily reached snorkelling sites around islands # 6 and # 9, and the cost runs between 200 and 300B. Note that the national park is not a tightly run organisation and this information is subject to change.