One of the properties of water is that it resists temperature change.
Organisms contain a lot of water. Besides sweating, panting, and all of those things that help cool a body, just the fact that the organism has a lot of water slows down the rate that the organism changes temperature.
Organisms that live in aquatic environments do not have to adapt to the extremes of temperature that organisms exposed to the air do. In summer the temperature of the air may change many degrees while the water may change just a few degrees. In freezing temperatures the water is still above freezing under the ice.
