Aquatic organisms are living beings that thrive in water environments, including oceans, rivers, lakes, and ponds. They can be classified into two main groups: marine organisms, which live in saltwater, and freshwater organisms, which inhabit freshwater bodies. Examples include fish, amphibians, crustaceans, and various types of plants like algae. These organisms play crucial roles in their ecosystems, contributing to nutrient cycling and serving as food sources for other animals. They have adapted to their environments in various ways, such as developing gills for breathing underwater or streamlined bodies for efficient swimming.