Try these guys:
http://www.travel.reperages-thailand.co ... .html#progQuote:
Hobie Cat Raid around Koh Phan Ngan (Gulf of Siam)
Hobie Cat expedition in the waters of the Gulf of Siam with navigation around the island of Phan Ngan and Samui for the discovery of the white sand beaches, the underwater floors, and the verdant coastline. The fleet is composed of 5 Hobie Cat 16 equipped Raid, and is supervised by an French-speaking guide.
Or this outfit:
http://www.island-cruises.org/sailing-on-koh-tao/I sailed with this outfit in 2003 when all he had were beach cats, but it looks like he's moved on the bigger boats. Contact the owner, Kunta, and ask if he can recommend beach cat rentals in the Koh Samui region:
http://www.sailing-in-samui.com/about-us/Good luck.