I booked a snorkelling day trip from a travel agency in Chaweng where you get picked up from your hotel and transferred to the Bang Rak port near the Big Buddha. From here you board the Seatran Discovery boat to Koh Tao where we then went to the small island of Koh Nangyuan which is just breath-taking with shallow crystal clear waters. You can do some snorkelling here where the shallow sea is full of wonderful tropical fish and then you can climb up a hill to get some great panoramic views of the island and sea. You then transfer back to the boat for lunch which was good - chicken curry, veg, rice and fruit and then we did some more snorkelling off the north tip of Koh Tao. You then head back to Koh Samui. Great day out which I got for 1600 baht whereas Seatran Discovery offer it on their website for 1700 baht.
Take a trip with a boat to the different small islands and coral reefs outside Koh Samui. It's a great relaxation snorkeling in crystal clear and swiming with the fish.