I spent approximately two weeks in the south of Koh Tao at Chalok Baan Kao at the tail end of a one month backpacking trip around Thailand with a brief stopover to Cambodia.
I had a fantastic time there, not because the diving was mind-blowingly spectacular but because the atmosphere and the people just have such a passion for diving and having fun. The diving was pretty good though.
There's a pretty good diversity of creatures and corals, with some great schools around the two pinnacle sites (Southwest and Chumpon). Decent snorkelling to be had too, for surface dwellers. A whale shark eluded me by two weeks time. Getting towards the end of the season the seas did kick up a bit. Visibility was good, but not great, 30-50 feet I'd say. Water temps around 26C/80F. Air temps slightly higher.
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