I went to the Similan & Surin last November with World Wide Dive and Sail. They also go to the Burma Banks.
The Web site is:
www.worldwidediveandsail.com
The owner is a Dutchman who has been teaching diving in Thailand for a decade. The boat, the "Sampai Jumpa" (See you later in Bahasa Indonesia), is a beautiful white sailboat. And the food was WONDERFUL! (Thai and Wester. I ate way too much.)
This is one of the more affordable liveaboards--so no luxury here. But it's a lot for the money, and it's not "dorm style" like many of the cheapies leaving from Koh Lak, though there are no en suite facilities. (But you get hot water!)
Very professional and experienced staff. New equipment too, in case you don't have your own. Great dives. Hoping to go with them next year to the Burma Banks.