Why Is Ice Floating On Water So Important For Living Things?

Because if it didn't it would kill the fish, crabs, turtles and other creatures that live underwater. This most fortuitous for our aquatic friends because other substances (like metals) become more dense when they change phase and  go from liquid to solid form.  Water is special because when freezes it becomes less dense than when it was in liquid form. Ice floats because it is less dense than water.