What is the most important property of water for protecting fish in a shallow pond on a hot summer day

 Low oxygen content.

Explanation:
There are many dangers to the survival of fish in shallow water that depend on the properties of water.
1. The oxygen content drops during the winter and also in hot weather. Fish cannot survive without enough oxygen in the water.
2. High evaporation rates can drain the pond.
3. Fortunately, water has a high thermal mass, so overheating the water to the boiling point is not an issue.

Of the three reasons given, lack of oxygen is the most critical.