@ZeldaPY
Thinking about it, but every single Zelda game has had a item that's needed to be found to swim underwater/swim at all.
-Zelda 1 & 2 no swimming at all
-ALttP, Link's Awakening, Minish Cap, ALBW, and the Oracle games have the flippers.
-The N64 games have basic swimming but underwater stuff requires extra items like the Zora scale to dive deeper, the Zora Armor/Iron Boot combo, or the Zora Mask.
-Twilight Princess has basic swimming but only allowed diving with the Zora Armor.
-Skyward Sword again had basic swimming but diving was only accomplishmed through the water dragon scale.
Like every game requires something extra to swim underwater so I actually wouldn't be surprised if we do get something like that in TOTK. Otherwise I don't really get the point in adding water to the Sky Islands.
Might just be Hopium on my end but I really hope they have some hidden suit or something that lets us swim.