To take a sunbath.

No.

It is because of density.

Density is the measure of mass present per unit volume.

Lesser the density, lighter is the object.

Now, every water molecule has two hydrogen atoms bonded to one oxygen atom.

However, besides being attached to its own oxygen atom.

A hydrogen atom is also attracted to oxygen atoms of other water molecules.

Forming hydrogen bonds.

In liquid state, the hydrogen bonds are not strong.

They constantly break and reform.

Thus, the molecules get arranged in disordered patterns which leave very less empty spaces.

However, when temperature drops below 4 degrees Celsius and ice begins to form.

The hydrogen bonds become strong and stable.

Forming hexagonal patterns which have a large number of empty spaces.

These large empty spaces make ice less dense or lighter than water, causing ice to float on water.