shore diving just north of Duluth we saw weird Ripples in the water, very similar to watching Illusion of water on a hot road surface. not sure if this is common or what. surface water was very warm (60 deg?) and got colder quickly.
My guess is that you were actually witnessing warm and cold water mixing. Google water turnover and read through highly technical explanations of water stratification, mixing, seasons, etc.
We dove at Brighton Beach Sunday, and the surface temp was 68 degrees, with a VERY definite thermocline at 22 feet, which you could actually see. Such a defined thermo is pretty unusual for the Big Lake...usually it's just plain cold, from top to bottom. Winds have been fairly light the past few days, so that might explain the thermo.