I live here in Switzerland and like to welcom you for kitesurfing. Silvaplana is the best spot in CH during the summer. When the wether is good, there is a big chance to have strong termic winds, that means 5-6bft. But take into consideration that the lake is in the mountains, about 1600meter over see level, the air is thinn and you need bigger kites!
The only spot where Kitesurfing is alowed at lake silvaplana is close to the sport center mulents, located in silvaplana. You will see the kites on the beach. Usualy on weekends the place is a little bit busy, i.e. last weekend we counted up to 60 kites in the air! But no worries, if you feel bad, go upwind on top of the windsurfers and you have the hole see sharing with me and some 3-4 other kiters
The water is about 16Ã‚Â°C and the air is not very warm also!
Have a nice time in Switzerland...