I been in Sihounukville in April 2007 after windy winter season in Vietnam.
The bay is nice, fine sand, flat warm water (almoust HOT, no refreshing in April ) but NO wind.
If you go left from public beach you can find "surf beach" , where few local expats store small sailboats, few windsurf with huge sails , and I also see 1 (one) huge ( 16 or 18 m) private C-type kite in storage.
Locals said that wind usually VERY light, almoust not enought for flying kite, but good for small sailboats.
No WS or kiteschool here. Just sailboats trip offer for tourist.
If you want kiting- just visit Vientam ( Mui Ne or Vung Tau), it not so far away from Cambo.