There is another wikitravel site regarding the same island:http://wikitravel.org/en/Ko_Yao, older and better informed.
"Koh Yao Noi" (engl. long, little island) is located in the Bay of Phang Nga, between Krabi and Phuket in southern Thailand. It is one of the biggest islands within the Phang Nga Bay archipelago and easily accessible from both Phuket and Krabi Provinces and presents some of the most beautiful sea scenery in South East Asia. Taking the public ferry or a private longtail boat and you can reach this paradise within an hour.
Most of the island's 4,000 inhabitants make their living through the rubber tree and fishing industry. A little rice farming and some fruit, palm and coconut plantations are evident around the island and boat building as well as farming techniques here have been passed from father to son. Being here, makes your life special.
The majority of inhabitants are Muslim and speak one of the southern dialects of the Thai language. Due to the upcoming number of foreign visitors, English is also well known around the island.
Like everywhere at the Andaman coast, the best time for travelling is during November and April. In May, when the low season starts there are even less tourists than in high season on the island and slowly the rainy season starts. From July to October, you have short but heavy rain falls mainly throughout the evening. Anyway, the location inside Phang Nga Bay protects the area from the strong Monsum winds most of the time.
The island is easy reachable from Phuket, Krabi and Phang Nga Province. The best way is to come with one of the public ferries or to charter a private longtail boat to the island. The boat ride is between 40 minutes and 1 hour 20 minutes, depending on your starting point. From Krabi and Phang Nga are just 1 or 2 ferries per day, but from Phuket you can get a boat more or less all day long, starting 7am until 5PM.
As there is almost no traffic on Koh Yao Noi, you can just rent a motorbike and explore the island on your own. As it is quite a tiny island, you can also get sportive and rent a bicycle.
There are also some tuk-tuk's for rent, which can take you to almost every place.
Kayaking, Rock Climbing, Snorkeling, Diving are just a few activities. There are also a lot of beaches which invite you to take a dip in the refreshing, clear water.