Just got back from Thailand and was disappointed to find out that the sea is totally overrun with large jellyfish right now - about one every 5 - 10 feet. If you're planning to snorkel - forget it - unless you don't mind getting stung repeatedly. Not life threatening, but nevertheless not enjoyable either. NO mention of this in any travel books we read :( A lot of people were going out on snorkelling trips, getting in the water for a minute, getting stung and calling it quits for the day. Patong Beach side islands were better as long as one stays relatively close to shore.
It was a total drag for me because I spend half my vacation in the water usually. Diving is fine - actually exceptional - but don't plan to float around on the water's surface.
I asked if this is common ocean conditions for this area, and the general response was usually they are not this bad around Dec/Jan. I wonder if that's true?