Expect a water tempertature not above 19/20°C.
For winds, check the very reliable windguru :
http://www.windguru.cz/fr/index.php?sc=48488
As of bars / clubs, it would depend on your age (people can be very young in the summer) but I recommend :
- Baignoire de Joséphine (aka "la B.J.) : 16 allée des alignements, 56430 Carnac
Atmosphere is regatta oriented with people from the "French sailing Mecca" (La Trinité sur Mer, few km south of Carnac)
- Whisky Club : 8 avenue des Druides, 56430 Carnac
For restaurants, I would suggest to go to La Trinité, on the harbour.
Carnac is quite large. Having a car there is better. Be careful to police checks if you are drunk though.