When a lake freezes, how does the density of water prevent all of the living organisms in the lake from freezing as well?

Because ice is less dense than liquid water (because the water molecules for a new, large structure as they cool down), lakes freeze from the top down even though the coldest areas are at the bottom--cold water rises when it begins to freeze and accumulates at the top in a sheet of ice.

Explanation:

The ice provides some insulation the water below, so that there is typically a significant portion of liquid water under the ice sheet, unless it is extraordinarily cold for a long time. If lakes froze from the bottom up, they would be more likely to freeze completely.

it gets less dense

Explanation:

That is why ice cubes float in a glass of water. It also takes a lot of energy removal to freeze water, and the air is usually much colder than the earth at the bottom of the lake.