Originally posted by dr4296:

... but my friends kept saying they thought they found some in waters they'd already traveled through that were previously empty. But now I can tell them that that's incorrect.

Yes, you can but I fear you may be wrong.

Many other creatures have fixed spawn points which may not produce a creature the first time a viking passes that location. That does not mean a creature will not spawn there in the future.

I have a vague recollection of sailing past a leviathan in a location that was empty when I traversed it on previous voyages.

Normal spawning in the game is random. Spawn points do not automatically produce the associated creature. In my experience, a troll cave does not always contain a troll although that is the way to bet. Also, in my experience, once a viking has killed a troll in a troll cave another troll will not spawn inside the cave. Leviathan spawning may work in the same fashion.