The 11 Similan Islands make up Similan Islands National Park, located in the Andaman Sea, off Thailand's west coast in Phang Nga Province. The largest, Koh Similan, is about 2 square miles, and is made up of huge boulders, including one that seems balanced on the island's highest point. We think it looks like a monkey's head, but it's known as Donald Duck Rock.
There's no dock here and it will be a "wet landing" when you come into the shallow beach. You'll have to hop off a tender or Zodiac into shallow water. You will probably be taken to Koh Similan, which has information placards about the local flora and fauna, as well as restrooms, a couple of vendors selling drinks, snacks and a few souvenirs, and a campground.
From 11 to 11:30 a.m. and 2:30 to 3 p.m., there is a massive invasion of day-trippers to the island -- so it's best to avoid going ashore during those times. If your ship offers the opportunity, go on a snorkeling trip instead; there are also day-tripping snorkelers, but the prime area is large enough that you won't be on top of each other.