Water has the property of anomalous exp i.e, it's density is max at 4 degrees and decreases below 4 degrees.Therefore when temp lowers and reaches 4 degrees the upper layer of water becomes max in density and goes down . The exposed layer is lighter ( as it has to face a temp below 4 degrees ). This process goes on and the surface freezes at 0 degrees but aquatic life remains alive at the bottom at 4 degrees
Most bodies of water with fish in them have an aerator in them to keep one section of ice open and allow oxygen in the water in the winter. If the ice completely freezes over the fish will suffocate and die.
Well water in lakes and oceans almost NEVER freezes completely (of course there's some exceptions but this is the case 99.99% of the time!). One reason is due to waters ability to retain heat and not freeze due to that. We also know water has a relative density of 1. However water is MORE dense at 4degrees Celsius thus, water doesnt freeze at the bottom (0degrees C is freezinbg point)
but goldfish and feeder fish better known as carp can come back to life after being totally frozen. that's why people stock small ponds with these kinds of fish because they can come back after being totally frozen.