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I enhanced the octopus pic. The background looks like some Mario 3D Land-esque cliffs. I thought maybe it might be from Mushroom Kingdom U when water floods a world but none of the worlds look like that. The water and cliff color are wrong for Wuhu Island and Delphino Plaza. I'm not sure what this level is.
Game and Watch's final smash actually adds a gray-ish filter to the world: https://www.youtube.com/watch?v=npk7Fk8e40g#t=768
That said, the 'white patterns' in the water don't match wuhu anywhere OR delphino.
It only bears a passing resemblance to the Pirate ship stage:
The color would be accounted for in the final smash, which adds the gray filter, but adding a transparency to the water would be an interesting design choice.
It's possible that if swimming is removed (and replaced with a slow sinking), then they'd make the water transparent so you can see how far below the surface you are. That would kinda ruin the Wind Waker Aesthetic, though.
BUUUUUUUUUUUUUUUUUUUT, you can see the room flooding in the Mushroom Kingdom U stage here: https://www.youtube.com/watch?v=iCfRCTlULs0#t=132
It's a bit hard to make out, and we don't have any direct-feed footage/shots afaik, but I'm pretty sure you can see the patterns in the water there.
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So yes, I think you're right. NSMBU.