It is essentially a transmission cable for carrying electric power below the surface of the water. Such cables are usually carrying electric power beneath salt water - hence the name subsea, or submarine. However, such cables can be used beneath fresh water also, e.g. like they do to connect the mainland with the islands in the St. Lawrence River in US. The installation of such cables is a hard, tedious and super expensive work and, if necessary, they can be buried as deep, as the mountain Everest is tall.