This was an amazing trip on a liveaboard boat with some of the best coral we've seen in the past 20 years of snorkeling throughout the world. Owner/operator Ralf does a great job of finding hidden snorkel spots with plenty of colourful coral and fish but without masses of people. Highly recommended - but definitely not for softies (the exercise mats on the bunk beds take a bit of getting used to).
The Similan and Surin islands are treasures that need to be protected for all time -- we were gratified to learn that Thailand has protected many of these islands and amazing reefs in National Parks - see Similan Islands and Surin Islands. If you are using the Chrome browser, you can also check out this Google Earth link for a splendid 3D view of the Surin islands from above.
Unfortunately, the islands are suffering from too many tourists. At one point there were up to 7,000 visitors per day. The problem of overuse has prompted park authorities to limit the number of tourists in the Similan Islands to 3,325 people a day.
The ideal time to visit these Islands is December to April when it is the dryer with clear waters making it best time for scuba diving and snorkeling. You can even stay overnight in one of the campsites with tents and bungalows on Surin Nuea Island.