I can't say about the food but I don't agree that the rock face is making the water is too cold.

Rock acts like a huge temperature sink that moderates the temperature. Because of it's mass, it is slow to heat and slow to cool. It could make the water slower to cool in the early winter and slower to warm up in the spring; but in mid winter, I think the rock below ground at the level of the water would be close to average temperature at that depth in the same area.

Wind speed has no effect on the actual air temperature. It does make things cool or warm faster. So the rock surface would get a bit cooler or warmer faster, but this temperature needs to be transmitted to the below ground to affect the water temperature at depth. At any depth, that rock is surrounded by the ground which moderates the temperature of the rock. If the rock is colder than the surrounding ground as you believe, the rock gets warmed. So as the cold air temperature moves through the rock underground, the surrounding ground moderates the temperature of the rock making those temperatures equal.

The ground is virtually an unlimited temperature sink. It can warm that rock to ambient ground temperature without any problem.

Therefore, I believe the temperature of the rock next to the deep water is exactly the same temperature as the soil bottom that is next to another hole that is just as deep. Temperature changes in the air are not instantaneously transmitted under the ground by rock. It takes time, and time moderates any temperature changes.

Your temperature theory is easy to test. Put a thermometer in the water to see if it is colder than the water above this area. Since cold water is denser that warm water and sinks to the lowest spot, you need to test the water at the same depth in both places. An infrared thermometer could be a way of easily testing the surface water temperatures.