Tectonic Activity Warms The Water
The water in the subglacial lakes is about freezing but is just warm enough to flow around in a liquid state. This is because the immense pressure of the ice – which is often over a mile thick – is so intense that it warms the water on the very bottom.
Additionally, the warmth of magma that flows underneath the earth’s crust comes from below the lakes and causes the water to warm, just enough to be in a liquid state. So they aren’t exactly hot springs, but they exist in a sub-frozen state that only enables them to flow around and be considered freshwater lakes.