"pod morzem" (Locative), but that's semantically weird because being literally "under the sea" would mean... that you live somewhere inside the planet?
I don't know how it's really translated, but I'd go either for simple "pod wodą" (underwater), although you could point out the same semantic problem here, or maybe "na dnie morza/oceanu" (on the seabed/ocean floor).
Haha, this always happens to me. I've never actually seen that sentence somewhere before, so I had no choice but to go for literal translation, and it ends up being pretty much incorrect or not really used in Polish.
I also had a feeling it was Locative, but I couldn't remember.
Thanks for the pointers!
Does the expression mean "yellow mushroom" as well - it would in Bulgarian and many other languages.