Hi, i spent quite some time on Phangan and learnt to kite there: wind is not very strong but the island is like paradise. You will need a big kite (12 or bigger mostly). Kiteschools are located on the south-west side of the island, between ThongSala and HaadRin. Windy season starts in December until March (south-eastern wind mostly) then turns to North wind in the off-season.
Foil is difficult (maybe impossible?) inside reef because of tide. Only flat water inside reef, some rocks and urchins and seaweed. Might need some shoes (you can buy them locally).
Best spot is IMO where Breeze Kiteboarding is located (check Facebook), ask for Dani V. the owner and operator, really nice guy and very knowledgeable about conditions, he's been operating for years and years, AND speaks perfectly english. They have Ozone gear for lessons and rentals and sales available. Oh and every year before the south-wind season starts (anytime from november-december) there's a short period when all the schools move to the North-side of the island to Malibu beach (village name Chaloklum) for some "storm riding" .
All in all a great destination if you don't mind not-so-strong-winds. Everything else on Phangan is beyond great