Freshwater is less dense than seawater. Warm water is less dense than cold water. This is essential to the ventilation, or supply of oxygen, to the deeper parts of the ocean. Ice is less dense than water, so ice floats. Ice and its snow cover insulates bodies of water and helps keep the underlying water from freezing.
Water is a good solvent .
Water can even dissolve rocks, the source of many of the salts in seawater. This allows plant growth to substantial depths. Water does gradually absorb light. This prevents plant growth below about 200 m depth even in the clearest ocean water. Absorption of UV light protects sensitive organisms.