Which of the following terms best describes organisms in an ecosystem that are dependent on water?

The term that best describes organisms in an ecosystem that are dependent on water is "aquatic." Aquatic organisms are those that live in water environments, which can include oceans, rivers, lakes, and ponds. These organisms rely on the aquatic environment for various aspects of their survival, including obtaining nutrients, reproducing, and carrying out metabolic processes.

While "freshwater" refers specifically to organisms that live in water bodies with low salt concentration, such as lakes and rivers, "aquatic" encompasses both freshwater and marine environments, providing a more comprehensive term. The other options do not accurately capture the dependence on water; "terrestrial" refers to land-based organisms, "submarine" is generally used to describe underwater vessels or activities rather than organisms, and thus does not pertain to an ecological classification of living beings. Therefore, "aquatic" is the most inclusive and accurate term for organisms thriving in watery ecosystems.