I think it refers to an inland body of water with no access to the sea, sort of like a salt lake [1]. (I upvoted because it's an interesting question. But it would be better received if phrased plainly versus sarcastically.)
The perils of thinking people receive things objectively. I didn't think it was sarcastic; it's phrased about as neutrally as I can imagine.
Try “what is an inland lake?”
It’s the question, plainly. You aren’t levying a guess against a contrived hypothetical (“offshore lake” isn’t a thing, and could objectively be parsed as facetious).