Came back from Koh Tao and wanted to put some thoughts down to help other travelers on some questions that I had before I went to Koh Tao. We stayed in Sensi Paradise, so you can read my review of this place separately. Basically, the layout for the area where we were staying is this:
Town/pier -> small resorts -> Sensi Paradise -> Charm Churee Villa -> Bamboo Huts -> small huts whose name I don't know by Sai Nuan beach. There are small resorts sprinkeled throughout, but it's about 5-7 minute walk from Sensi to town, and about a 15-20 minute walk from Sensi to Bamboo Huts, all along a relatively easy to follow path. I'm glad we stayed in Sensi, otherwise I probably wouldn't want to walk to town at all, if I had to walk back 20-30 min at night.
My biggest question was how good was the snorkeling. We love to snorkel, and went to Koh Tao mainly for the snorkeling. The snorkeling was pretty good, and the beaches were prettier than the beaches we were at in Koh Samui. There was snorkeling of smallish fish off of our beach in front of Sensi Paradise, which was pretty decent off the beach snorkeling. We had read that there was a great beach next to Bamboo huts, Sai Nuan Beach, with great snorkeling. The beach was ok, the snorkeling was not great and had some currents. We didn't mind exploring through the different resorts to get down to the beach, but I thought that the snorkeling in front of Sensi was better. The fish were small but plentiful and you could really see them up close. At all the beaches, you have to be careful when the tide is low (it was really low while we were there, but if you're careful, it means you're really close to the fish). You also have to be really careful about the small boats b/c they come in and out of the beach where all the snorkelers are.
We rented a small boat to take us snorkeling. I was confused on how exactly you go about renting a small boat if you don't want to go through your resort, but I think if you go early morning to the pier, starting around 8am, you can go and negotiate with the taxis. I tried to go at night for the next day, and everyone said just come in the morning. Or you can see them come in and out of the beaches, and negotiate then. Some of them have snorkels and life jackets, some of them have shade, some is just a basic boat. Be aware of what you are negotiating for. We are a family of four and 2500 seemed to be the going price (they take into account how many people) for the whole day. Someone on the forum said 2000, but only 1 boat agreed to it at that price for a max of 5 hours, and didn't have snorkels. People recommended doing the big party boats instead, since they will feed you lunch and it works out to be 600-800 a person (it is advertised all over the islands), but we preferred to rent our own boat so we can come and go as we please.
I think we followed the same itinerary as the big boats, but tried to stay a step ahead:
Shark Bay- saw 3 sharks swimming together at around 9am. We luckily missed the big rush, as there was 1 boat there when we came, and 3-5 boats there when we left.
Ao Leuk – was ok, didn't look special so the kids opted to take a rest.
Hin Wong Bay – big rocks around the bay, I thought the best out of the 4 stops, lots of fish, and pretty clams to look at (but small).
Mongo Bay – we ate lunch here, but the problem is while we ate, the big boats all came in so by the time we finished lunch, the bay was packed. There was nothing interesting in snorkeling, I barely saw any fish. One fun thing we saw when we ate lunch was a big blob hanging around the boat that was slowly changing it's shape. Turns out it is small fish which hang out together, shape shifting fish. We had a lot of fun swimming through the fish and watching them disperse.
Thailand is not known for snorkeling, and I think this is probably the best that Thailand has to offer. Doesn't compare to Maldives, etc, but the boat trip snorkeling is probably similar to what you'd see off the beach in good snorkeling spots off the beach in Maui. (didn't see turtles in Koh Tao though) Personally, I think it's a pain to get to Koh Samui compared to other islands, and you have to take an additional ferry ride from Koh Samui to Koh Tao. Or you have to do the ferry/bus/plane combo from Nok Air (which we took, even though it was tiring, it worked out fine. I would definitely do this rather than trying to coordinate myself to save some money. This way, if the ferry is late, then the plane will wait for you due to the coordination). I don't know if I'd recommend a vacation in Koh Tao just to snorkel, but we enjoyed our boat trip around the island, and enjoyed the beaches in Koh Tao. I don't regret going for our vacation. The thing I would do differently is check out the trip advisor restaurant reviews before going and stick with a known quantity. We just went into town to check out different restaurants, and thought the meals were just ok, including one bad. We didn't take taxis around, and just chose to stay close to the resort and town, in part based on forum advice against certain moterbike companies and the expensive price of taxis in Koh Tao. Felt more like a backpackers town than Koh Samui, and I think it may be a good place for back packers willing to stay in cheap non aircon bungalows on the beach. Just figure out how to keep mosquitos and bugs away, b/c there are lots. We ended up in the pool more than we normally do, b/c mosquitos wouldn't bite us there!