I found this. The warmest water temperature is in August with an average around 70.5°F / 21.4°C. The coldest month is February with an average water temperature of 36.9°F / 2.7°C.
Lake Superior has the coldest water temperatures. Summer averages 11.4 °C /52°.5 F
When I kayaked on Superior I always wore a wet suit just in case I ended up in the water.
I've seen some mighty waves on the lakes!