An aquatic cryptid is an creature, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that live in them can be categorized as aquatic (water) or terrestrial (land). Animals that move readily from water to land and vice versa are referred to as amphibians.
The term aquatic can in theory be applied to cryptids that live in either freshwater (freshwater animals) or saltwater (marine animals). However, the adjective marine is most commonly used for animals that live in saltwater.