The Gulf of Thailand has a number of dive spots worth visiting. The best are Sail Rock (the only 5 star dive site in the Gulf) and Angthong National Marine Park. There are a number of PADI certified dive shops that will organise fun dives as well as scuba courses for all levels from beginner to dive master.
Other watersports include wakeboarding in Chaloklum and wind surfing off Plaay Laem beach.
Most beaches also have places hiring out kayaks. It can be good fun kayaking in the stormier months from October to January when a bit of swell picks up. During this time you can body board in Thong Nai Pan. But you must remember the waves are not really for surfing, they dump and the undertow can be lethal.
Fishing trips can be organised from Koh Phangan out to sea to catch 20kg mackerel and 10kg barracuda. Shark and snapper are also possible to catch.
If you just want to see more of the island by boat then there are numerous companies offering boat trips around the island. They usually take in take such scenic spots as Than Sadet waterfall and the beautiful beaches of Thong Nai Pan.
Sadly a few beach resort areas now offer jet skis for hire. These toys for boys chug out polluting petrol fumes and ruin the tranquility of the beach.
Sometimes you see waterskiing on offer but this watersport hasn't reaｌly taken off on Koh Phangan.
Finally, in the high season (June/July) there is sometimes an outfit offering parascending on Thong Nai Pan Noi beach.