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. Water transmits sound well . Many organisms rely on sound for echolocation or for communication.