Sailors call this "dead water," a mysterious condition where a ship sailing in open ocean suddenly slows down for no obvious reason.
Now, scientists have recreated the effect in the lab and captured it on video:
Research has already shown that dead water occurs when an area of water consists of two or more layers of water with different salinity, and hence density – for example, when fresh water from a melting glacier forms a relatively thin layer on top of denser seawater. Waves that form in the hidden layer can slow the boat with no visible trace. [...]
It is the boat itself that initiates the wave – water from the layers below is dragged upwards to fill in the gulf its wake. That sets up an oscillation in the boundary between the layers, which gradually grows as the boat moves forward.
The wave gains size and speed until it, and the trough in front of it, eventually catch up with the boat and sapping its energy before the wave breaks against its side, Mercier says.
Link (with video)