I have ALL Tolkiens letters, and works on MIDDLE Earth on a giant PDF file…. I hit search for any word or phrase and can typically find what im looking for (some of us old boomers need to verify before we open our mouth or fingers) and NOPE no place is aqua-man mentioned or underwater wars / fights.
At the end of the First Age of Middle-earth, Beleriand was broken in the War of Wrath by the angelic beings, the Maiar, against the demonic Morgoth (a Vala fallen into evil). As the inhabitants of Beleriand, including masterless Orcs, beasts of Angband, Elves, Men and Dwarves, fled, much of Beleriand sank in the sea.
We could easily get to see this happening, as part of Galadriel’s visions, or as part of the regular show.
I don’t know how quickly it sank, but floating bodies of the dead I can easily see from that reveal.