Water is the medium for chemical reactions in body (Barney Harris, Jr. and H. H. VanHorn).
Water is the only pure substance found naturally in all the three states of matter i.e., gaseous state, liquid state and solid state. The density of water changes with temperature, as the temperature of water is lowered it becomes dense and at 4C it reaches maximum density, on further cooling the volume of water expands by about 9% to become less dense and form ice. Thus the ice floats on water and doesn't sink..(Explanation of the Anomalies of Water (1-10) - Shrinks on melting)
If water were solid when more dense in an artificial scenario, then the ice wouldn't float and sink to the bottom consequently the oxygen content in the water at the bottom would increase (Since at lower temperatures the oxygen content is increased) then the metabolic activity of microorganisms near surface would increase because of increased temperatures but face oxygen deficiency (Since at higher temperatures the oxygen content is decreased). Oxygen depletion is also due to buildup of organic materials from human activity, thus causing oxygen deficiency to the fish, invertebrate, plant and aerobic bacteria. Oxygen levels below 3 parts per million would mean a grave situation for all these aquatic organisms and an aquatic ecosystem imbalance. (Exploring Environment Water Quality- Dissolved Oxygen)
If the water was denser then ...