What professions do people need to master in order to work at sea?

There are many professions that allow people to work in the sea. People who are engaged in science can work in the sea. Namely, scientists studying marine life, fish (biologists, ichthyologists, etc.). In the sea “live” people engaged in fishing, ship transportation (captain of the ship, boatswain, personnel serving the ship and its installations, skipper, etc.). The life of the naval servicemen is connected with the sea. Oil and gas are produced in the seas (on the shelves). (geologist, drilling supervisor, oil and gas production operator, installers, etc.).