Chalok Baan Kao, Sairee, and Mae Haad
The three biggest beaches on the island. This is where you can go for a good meal, shopping, the majority of dive shops, lots of accomodation, and some spotty wifi. However, I would not recommend any of these for the actual beach. Dirty with murky water and lots of boats, these beaches are not great for swimming or snorkeling. Nor are they very relaxing and often the bars and restaurants play loud music. But if you are looking to go out at night, Sairee especially lights up with fire dancers and entertainment.
Note that you probably need a taxi or motorbike to get from one of these main beaches to the other.
This is a small beach off Chalok that can be reached by road or by walking to the far end of the Chalok boardwalk, taking a few steps through the water, and then walking on raised concrete pathways over the rocks. Here the trees hang low over the sand and are filled with hanging corals. The water is extremely warm, much like a hot bath, because of the shallow waters. This small beach can get a little busy on nice days.
The famous three tiny islands off the north end of Koh Tao. They must be reached by taxi boat, unless you book a tour. You can find a taxi on Sairee Beach and you can expect to pay at least 3,000 thb ($10) roundtrip person. Once you reach the island you will pay 100 baht and leave behind any plastic bottles and fins. In the mornings this beach is full of tours and day-trippers. Looking out on what is supposed to be clear blue water, all you will be able to see is the fluorescent orange lifevests of snorkelers. But in a few hours around 1:30/2:00 the beach will be much more empty and relaxing. In the mean time you can explore the islands on the raised pathways and climb up to the famous viewpoint. Bring sunscreen because the umbrellas sell out and there is little shade otherwise. Once the beach clears out, snorkeling and swimming is beautiful on any of the three beaches. Unfortunately the beach closes to all at 5 pm and you have to leave the island.
A rocky beach on the east side of the island, Tanote is known for awesome snorkeling. There is a resident sea turtle that usually pops out of the big rock around 5 pm. I also had friends spot multiple sharks and stingrays at this beach. If you dive here not only do you see the beautiful reefs, but you can explore a sunken catamaran and motorbike that are now teeming with sea life.
This is a small beach next to Shark Bay. Beware of the extremely steep hills on your way here (on taxi or motorbike). You actually have to park 5 minutes above the beach and stumble down the on foot to reach this beach. But it's totally worth it. There's one dive school/beach bar, a restaurant, and a handle of houses. There were times I visited this beach in the late morning and I had it to myself. The water is clear and bright blue, but this is not the best beach for swimming because the coral starts shallow (maybe 15 feet from the waterline) and stretches across the beach. But this means it is great for snorkeling as there is so much to explore.
Actually Sai Nuan is two beaches, split by huge rocks. The first is good for grabbing a bite to eat or getting accomodation (there's a few bungalows), while the second is a better beach. Beach swings, climbable boulders, a deserted taxi boat, and tall palm trees make this an ideal place to spend a day. The snorkeling is pretty good and the water unmuddied. I saw a lot of fish here, but not a big variety.
My personal favorite beach is relatively secluded, but still popular. Between Sai Daeng and Tanote, it is not hard to get to as long as you don't mind steep hills. Sure there are bungalows and a couple beach bars marring the landscape. Yet nothing compares to the clear deep turquoise of the water. This beach is great for swimming, with a sandy bottom that slowly deepens. Snorkeling and diving are also fun here because of the coral reefs that line either side of the bay. Order a coconut at one of the two beach bars that offer umbrellas since there is very little shade here. Honestly, I often would sit on the beach, my book laying forgotten in my lap, my gaze caught up in the blue of the sea.