Why doesn’t water freeze under a thick layer of ice?

The layer of ice has a lower thermal conductivity than water, so warmer water removes cold from the lower layer of ice and distributes it throughout its volume. If the ice layer is thick enough and there is a lot of water under the ice, then the water does not freeze through to the entire depth, if there is little water, then the reservoir can freeze. Therefore, the ice is thicker in shallow waters and thinner in deep places.