The most beautiful island I saw during our trip to Thailand was Koh Nangyuan. We took a longboat out there from the beach outside our hotel in Koh Tao, which cost 700 baht (21 USD) for the both of us, roundtrip. It’s a tiny island with only 1 resort/hotel. There are actually 3 small islands that are connected together by this strip of white sand. The tide was a bit high when we got there in the morning, so the connecting sand to the last island was covered in water. We saw one couple trying to get to that third island walking through the ocean with their bags held high above their heads. They seemed to make it without the luggage getting wet.
It was a pretty unique beach when you have water coming in on both sides of the sand! There were a lot of people snorkeling and scuba diving as well. You do have to pay a fee to enter this island (100 baht), and you are not allowed to bring plastic bottles with you. The food and drinks are a bit pricier at the one restaurant they have at the resort there. Also, you do have to pay for the beach chairs as well. However, it was worth the bumpy boat ride we took to get there. This would definitely be a must see place if you go to Koh Tao. I think you can take boats out here from Koh Samui as well, but it would be a much longer ride!