Other forms: marine biologists
A scientist who studies everything that lives in the ocean or sea is a marine biologist. Your little sister's love of dolphins may inspire her to become a marine biologist one day.
Marine, "pertaining to the sea," derives from the Latin mare, "sea," and a biologist is a scientist who studies living organisms. This scientific specialty focuses on plants, animals, and smaller organisms like plankton and tiny microbes that exist in salt water. A fascination with coral reefs, stingrays, tidepools, or kelp forests is a great place to start if you're interested in becoming a marine biologist!