Not vague. Translators thought ocean water will always remain stable, nothing is able to destabilize it.
More careful Translations such as NIV put it: “roaring and tossing of the sea.” (https://biblehub.com/luke/21-25.htm) Word for “tossing” is σάλου (salou), from saino; i.e., billow,” (swell outwards). Thus underlying thought is like balloon swelling, or water flowing outwards, as though ready to roll over, thus points to the modern terminology “sea-level rise.” Biblos Interlinear Translation (INT:) translates it as “of [the] sea and rolling surge.” “Rolling Sea.” (A Greek Lexicon to the New Testament, Edward ROBINSON (D.D., page 416) (A Greek Lexicon to the New Testament on the basis of Dr. Robinson ... by C. Robson).
Jesus’ original use of the wording “tossing of the sea” is another way of saying “sea-level would rise” as a result of pollution of air. (Revelation 11:18)
The same word is used in the sense of being "shaken, unsettled" as happens during persecution. (https://biblehub.com/1_thessalonians/3-3.htm)
That means sea-level rise was actually predicted, and was also being fulfilled in a monstrous way in our time. But "faithful and discreet slave" failed to discern it and thus to make use of it for their credit.
If JWs had done this well in advance, they would have been even more famous than that of 1914 teaching.
Now, there is no charm.