Different phases of water have different densities because the spacing between water molecules changes depending on the temperature and pressure of the water.
In its solid phase (ice), water molecules are held in a rigid crystal lattice structure, which causes them to be more spaced out and less densely packed than in the liquid phase. This means that ice is less dense than liquid water, which is why it floats on top of liquid water.
In the liquid phase, water molecules are less ordered and more free to move, which allows them to pack more closely together than in the solid phase. This makes liquid water more dense than ice.
At high pressures, such as those found at the bottom of the ocean, water can be compressed, which makes it more dense than at lower pressures. This is why deep ocean water is colder and denser than surface water.
It's important to note that these changes in density are due to the arrangement of water molecules, and not changes in the mass of the water itself. The mass of a given volume of water remains constant regardless of its phase or density.