Koh Phangan is the (in)famous party island where they hold the Full Moon Party.
There's a scheduled ferry service but it's also possible to buy a place on a speedboat, but watch out. Some of the boats are not properly registered, or may not carry appropriate safety equipment (like life jackets) and it's common for the drivers to be under the influence of alcohol (especially at the end of the night's party).
Here's a few tips on staying safe at the party too:
It's a great experience, but make sure you stay safe.
Phil from the safety hub about 6 years ago