Kayaks are avaiable for me hire on most beaches for Bt60 to Bt100 per hour. The obvious precautions should be observed such as covering your skin from the hot sun, allowing enoung time for long trips (sunset rides are not advised) and taking water to drift, it can be easy to go in one direction and very difficult to come back.
The west coast from Work Turn to Koh Ma offers some of the calmest seas and best beach hopping possibilities though if the seas aren’t choppy, pretty much anywhere is good. Other popular routes are between Thong Nai Pan and Haad Khuat (Bottle Beach) and also from Haad Rin it is easily possible to make it around to Haad Khontee and furtheron to Haad Thain and Haad Yuan but don’t depart too late in the day.