Unidentified submerged object
Unidentified submerged objects (USOs) are the lesser-known marine counterpart to UFOs. Like UFO reports, most USO sightings are either mundane, or impossible to verify. They are usually of the form "something glowing in the water", though the term sometimes is also used for the cases when a more traditional "flying saucer" decides to take a dive or rises from beneath the waves. *dramtic drums* If you have watched The Abyss or played X-COM: Terror from the Deep, you know what we are talking about.
In UFO books (and nowadays, websites) USOs are usually discussed together with ghost ships like the Mary Celeste, the Bermuda Triangle and mysterious collisions and accidents with submarines to reinforce the spooky mood and make you dread leaving the shore behind the horizon.
Note that bioluminescence is common in the ocean and there a lot of glow-in-the-dark critters that rise to shallower waters during the night to feed. Note also that during the Cold War, ramming or getting rammed by a Commie submarine could have far-reaching political consequences, so both parties tended to whistle innocently and pretend that nothing had happened. That didn't always work.