The best way to travel throughout the Similans is by liveaboard vessel. Much better than doing two-tank daytrips out of Khao Lak, being on a liveaboard gives you 4 or 5 dives a day, with you staying on the Islands' doorstep.
Black Manta is by far one of the nicest vessels offering dive trips in the world. Purpose-built for diving, all considerations have been taken into account while building.
Very professional crew, dive briefings were detailed, and and equipment issues were handled well, even if it wasn't a rental from them.
Offering much room and great food, booking a trip on BM will not leave you dissappointed.
Am already planning another trip with friends on the same boat.
The only way to get to Ko Similan is by a speedboat... I took the speedboat from Khao Lak and it took about 2 hours to reach the island..
The journey to Ko Similan depends on the weather.. On the day that I went, it was a cloudy day and the water was very choppy.. None of us got seasick but one of they guy who sat right in front of the boat got his back hurt when the boat was hit by a big wave... So, DO NOT sit right in front of the boat if you have back/spine problem.. The best is still to sit right at the back of the boat...
Our return journey was very smooth as it was a very clear and sunny day...
When You book Your trip to Similan, do it from the National Park head office at Tab Lamu pier, they will take care of everything - Speedboat and accomodation, You can choose if You like to stay at no 8 or no 4, but no4 is the only one with Bungalows
Many liveaboards depart from Phuket. Ask any dive operator for details. On my first trip I went on a boat called the Jonathan Cruiser. The journey was overnight and took about 8 hours. I did manage to sleep for some of this.
An alternative is to go from Khao Lak which is on the mainland north of Phuket. It is a quieter resort and closer to the Similans. The trip only takes around 4 and a half hours. I used the Sea Dragon Dive Center on my second trip.