I'm also curious if this is more a pond thing than lake.
Once the water warmed in my pond, about 20 or more sunnies were cruising just below the surface in a large school swimming in the same direction parallel to shore. A bass was swimming beneath them. It was a bright sunnie day with wind around 5-10 mph, water temperature I'm sure warmer near the surface at about 50.

Have no clue the biology of this almost daily visual.