One important thing about ice being lighter than liquid water is that this allows the oceans (and any other water masses such as lakes) to remain largely in liquid form.

If ice were denser than water, it would sink as it formed and a temperature gradient would exist from the ice at the bottom of the water to the surface of the water. Lakes would freeze from the bottom up and completely freeze over. Similarly the oceans might do that, if not for geothermal energy.

Instead we have the situation where water is densest at 4 degrees centigrade/celsius. As a body of water cools, the main avenue of heat loss is through the water surface. As it cools to 4 degrees centigrade, the temperature gradient is such that the warmest water is at the top, as the warmer water is less dense than the colder water below, facilitating heat loss. As the temperature of the water cools below 4 degrees, the densest water sticks to the bottom, with progressively colder water above it. No convection currents mix the water as the coldest part of the water is now at the surface. As the water at the surface freezes, it insulates the rest of the water below, slowing freezing down so that liquid water can exist at the bottom in the coldest winter, if the body of water is deep enough.

Why is this significant? Being frozen in ice is much more detrimental to life than cold water at 4 degrees Centigrade (the temperature at which water is densest). Lots of marine and freshwater life would perish every winter where the temperature dropped below the freezing point of water. Isn't the world we live in wonderful?