Captain reported in detail
The hero of the Titanic affair was Captain Arthur Henry Rostron of the Carpathia, the first ship on the scene of the disaster. A little-known fact about Roston (I'd never come across it until now) is that his memoirs include a detailed sighting of a "sea serpent." There are quite a few puzzling incidents like this in ship's logs, although they definitely tail off in the past half-century.
COMMENT: I think there is still some mystery behind the sea serpent stories. We might well find some kind of enormous eel or eellike fish has escaped the nets of science. (For a fictional treatment, see Arthur C. Clarke's The Deep Range).