* Marine Biologist: Studies marine life and ecosystems.
* Oceanographer: Studies the physical, chemical, and biological aspects of the ocean.
* Hydrologist: Studies the movement, distribution, and quality of water on Earth, including oceans, rivers, lakes, and groundwater.
* Limnologist: Studies inland waters like lakes and ponds.
* Marine Geologist: Studies the geology of the ocean floor.
* Marine Chemist: Studies the chemical properties of the ocean.
* Fisheries Biologist: Studies fish populations and fisheries management.
* Marine Engineer: Designs and builds structures and equipment for use in the ocean.
* Aquaculturist: Studies and manages the farming of aquatic organisms.
So, the best way to describe a scientist working in the water is to specify their area of expertise.
