Thalassophobia is the clinical term for a fear of the deep, dark ocean, and it seems to be a condition which all of us suffer from to one degree or another. It is derived from two Greek words meaning “fear” and “ocean.”
We are all evolved from the ocean, yet we are also separated from its environment by millions of years of evolution. Being surrounded by darkness and water and alien creatures evokes a primal fear in all of us—which is why underwater movies form a distinct genre of horror.
Source: 19+ Underwater Movies: The Art of Aquatic Horror – Creepy Catalog