This is the MacArtney 11kV, 400 amp wet mate connector, a plug "eliminates the need to bring cables to the surface by wet mating and de-mating cable infrastructure, wave energy converters, tidal energy converters and floating wind turbines." I just find it fun because it kind of looks like a regular 120v plug.
MacArtney's wet mate connector shortens the time needed for connection and makes it possible to operate in waters with limited time windows. This is particularly important for renewable energy devices where changing tides or wave action are often associated with marine renewable deployments. The MacArtney 11kV wet mate connector provides safe and reliable connection to the power grid and addresses a number of fundamental challenges in building and maintaining marine renewable installations. Development of the connector was funded by ETI and testing witnessed by DnV (Det Norske Veritas).