The density of water determines its buoyancy (plankton is life in the water column, streamlined body shape in active animals). The temperature regime is smoother, that is, there are no excessively high and low temperatures. Limited oxygen supply. There are deaths in reservoirs for various reasons. The salt composition limits the distribution of freshwater and marine life.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.