For those who plan to go to Ko Tao and Ko Phangan from Ko Samui when you land on Samui look out for a counter that says to Ko Tao and Ko Phangan its run by a company Lomprayah. You can buy the tickets there but before that can check the ferry schedule here
http://lomprayah.com/E/index.htm
There will be someone who will ask you to wait for van to bring you to the jetty.
If you paid for the ferry at the airport you will get a voucher and need to exchange the voucher for tickets at the jetty
If you have time to kill and hungry MUST EAT here at the Harbour View Restaurant. The Thai food is great, very authentic taste not tilted for angmoh. I have to say I've not had such nice krapow moo and tomyam soup for a long time.
Kra pow moo sap
Tomyam Gai
Gaeng khiaw wan
Khana
But the price is a little angmoh, above meal 1,000 naht with 2 small bottles of Singha a coconut and a Magnum ice-cream