I'm going to be in Trang in late May 10 and I'm wondering about the ocean conditions down there at that time. We will be staying at the Anantara Si Kao resort and plan to take boat trips out to some of the nearby islands such as Koh Mook and Koh Kradan. I visited Phuket in June one time and the sea conditions were really rough due to monsoon weather. Does anyone know what the typical weather conditions are like for Trang at this time of year? Should I expect windy/rough seas for my boat trips?

I believe it's still going to be monsoon season (or close to the start of it), so I'd plan on rough seas and weather. I don't know about Trang specifically though :(

Weatherunderground.com has an almanac feature that gives you the averages, highs/lows and precipitation rates for a histroric date, so you can start there and get a pulse for May.

(Trang, Thailand Forecast : Weather Underground)

Trang is on the same side as Phuket that is on the Adamman sea so the monsoon will also affect that area. Went there before in NOV 2000. Not many foreign tourist go there and you mus visit the emerald cave which is in Koh Mook