Perhaps you asked that question because most streams and lakes where you live are covered with ice in the winter. However, very few lakes and large streams freeze all the way to the bottom. Ice provides pretty good insulation against heat flow. And since the ice always floats as a layer on top, the water underneath is protected and doesn't freeze.
You also might think about how neat it is that ice does float. If it did not, then lakes would freeze from the bottom up, and that would be bad for the fish.