The best time to visit Koh Yao is between November and February or at the latest, March. The days are very clear with temperatures hovering around the 80°F / 25°C.
In the evening the temperatures drop a little by about 5 degrees making for a relaxing time lounging around in your luxury resort or on the veranda hammock something to remember.
During the monsoon rainy season some of the boat services may be cancelled or delayed. The car ferry from Jian Warnit Pier has more chance of running than the longtail boats. But this is the May to September time, when it does rain quite a bit.
In amongst the rains there are clear sunny skies. The humidity rises at this time of year, which makes the jungle especially steamy. A day out amongst the trees would almost certainly mean a perspiring day.
The yachting potential is great in the Phang Nga Bay with plenty of moorings available on the both coasts of Phuket and here on Koh Yao Noi.
The weather obviously has a strong influence on the activity so checks should be made in the low season for the latest weather reports.
To some extent Phang Nga bay is sheltered from the worst of the storms, but advice is available from the excellent Thailand Meteorological website http://www.tmd.go.th/en/
The site is in English and gives storm warnings, and all the relevant information for anyone whose activities are weather dependent.
The Paradise Hotel and Spa offers anchorage facilities for visiting yachtsman, with an invitation for lunch, and drinks.