A lot of the underwater raid stuff was used in Firelands. I think the whole "flying" part of Alysrazor was supposed to have been an underwater chase initially.
I think this could be made to work but Blizz will likely never do it after having scrapped it once before. It's a shame. EverQuest managed it in Kedge Keep and Plane of Water (although EQ was notoriously bad at hit detection on the vertical axis). I'd be interested in seeing an underwater raid tier.
Currently playing Borderlands 1 remaster. Amped for Borderlands 3.
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Most WoW players couldn't handle a Phinigel Autropos fight.
I think most people have said it but Vashj'ir should answer your question. It'd be among the worst received boss battles. Which I assume is part of the reason they scrapped that companion underwater raid that was originally supposed to be released with Firelands in 4.2. Somewhere on these forums you can see the raid it's in the files and looks cool but noooooooope.
That being said if they did a Throne of the Tides style raid or whatever I'd be super stoked. That place is just plain pretty. It has all the benefits of being underwater and none of the "this game was not built around 3-D combat" set backs of actual under water combat.