How is it possible for aquatic life to survive extreme freezing temperature in the winters?

If the water is deep enough to not freeze solid, the ice layer on top acts as an insulation layer. Also, water is the most dense at 4 degrees C (about 39F) and this layer of water is at the bottom in a cold body of water or deep ocean. Animals also have adaptations to the cold. Some simply become much less active, being cold-blooded, and hang out in that "warm" water layer. Some have biological antifreeze. A few animals simply freeze solid (there's at least one frog species that does this, though I believe it's not totally aquatic) and then thaw out in the spring.