There have been times when Earth did not have ice caps at either pole, and other times when the polar ice caps covered much of the northern and southern hemispheres and glaciers formed on all the continents. During the last ice age, which ended about 10,000 years ago, snow, ice, and glaciers covered much of the northern hemisphere. When water freezes, it fills more space than when it is liquid. Water that seeps into cracks in the ground, like the fault line on which Loch Ness is located, expands and deepens the crack when it freezes. Geologists think the glaciers that formed where Loch Ness is now are responsible for the extreme depth of the Loch.
Ice is less dense than water, so it floats on top of water.
It's possible that the ice never went so deep that there was no water for Nessie to swim in. So she might have survived the last ice age.