Being a small tropical island, Koh Tao has, exept diving, a lots of water based activities available.
There are daily around the island snorkeling trips available through numerous travel agencies or resorts or you can be independent and hire your own long tail taxi boat for a full or half day. You can explore Koh Tao’s bays and coral reefs at your own pace. You can spend a few hours on the beautiful Koh Nangyuan, apparently the only place in the world where there are three islands connected by a sandbar. Any of water taxi’s will take you there from Sairee beach and pick you up at an agreed time.
Feel like you need some physical activity? Why not rent a stand up paddle board or kayak? The sea conditions make this ideal with low to zero waves and weak to mild currents around the island for most of the year. You can rent them from most of the beaches.