Visiting the Angthong National Marine Park is a good day trip, but choice your tour guide carefully.
First, it takes about one hour and forty five minutes by boat to get there. We enjoyed a light breakfast (coffee, bananas and pastries) on the way and caught some morning sun. Our tour guide was Samui Island Tours and they were very unorganized. They couldn't keep the groups straight and we were actually scolded by one of the guides because of their lack of communication. I'd go with Blue Star even though it's a little more money.
Also, skip the snorkeling. It's horrible. There is nothing to see. The kayaking was OK, but again this tour group was unorganized and you didn't have much flexibility to kayak without being in a tight group.
The trip back to Koh Samui was pleasant and great for tanning.
Not a bad trip, but try out Blue Star, the higher rated tour provider on the island.
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