Since Kaido is apparently a fish, does this open up the pandora's box of "aquatic zoans" then? How does it even function with the DF weakness to water? Is Oda breaking his own rules or has he just been misleading us all along deiberately?
Now the idea of an actual Water Fruit doesn't seem as ridiculous imo.
Well kaido in particular has the added benefit of being a mythical fish. And so of course had breath under water like all fish but as far as being unable to swim, maybe it doesn't matter
Think of it like this
Dragon form kaido doesn't actually move. Like kaido doesn't have wings or something that flap up and down to make him move. Kaido uses clouds. The clouds just telekinetically move kaido through the air as long as he's conscious to move them with his mind.
And since these clouds already make no sense given they can lift islands, it's not far fetched to imagine they can exist under water too.
So Oda has found away to bypass the weaknesses. Kaido can breath under water I account of being a fish and we know even fishmen wit devil fruits breath under water. And kaido's body can't swim of course but Dragon kaido doesn't actually use his body to move around. It's his clouds that move him. So as a dragon, he can still move under water by clouds and breathe under water. Oda pulled it off.
And no, sea water doesn't shut off devil fruits. It weakens the body to prevent movement and then you drown. If Kaido's body weakening doesn't matter since his clouds he controls with him mind are what moves him and he can breathe under water as a fish, then there we have it
Land sea or air.