A material is considered biodegradable in water if it can be assimilated by the microorganisms present, with no negative impact on the environment. However, a plastic may be biodegradable in seawater but not in freshwater, and vice versa. It is important, however, not to confuse biodegradability with water solubility, which are two different processes.
The main advantage of using biodegradable polymers in water is to limit plastic pollution in rivers and seas.