I just came back from Samui and my wife and I took a snorkeling trip from Samui to Koh Tao as we enjoy to snorkel in warm waters :-) We picked up different brochures at the hotel and we decided to go for LINK Seatran Discovery (see http://www.dial-samui.com/tours2.htm ) and we paid about 1700 THB per person. The problem was that this big boat is also used as a regular transportation boat to Koh Pa Ngan and Koh Tao before it proceeds to the snorkeling so we got there 1 hour behind schedule which only gave us 1 hour to snorkel from the boat. We were also a bit shocked to find out that fins were not included in the package and it was not possible to rent either on the boat. Then we were dropped of at Koh Nangyuan and the return was 1 hour earlier than stated in the program. We started on the hometrip at about 2 pm and we came back to the pier at 5.30 as it took some time going to pick up people at Tao and Koh Pa Ngan again.
So if you are into snorkeling I would recommend that you find another company because this was a waste of time.