Had something like this happen to me, when I started my namekian, I got on a bus... and got off on a bridge and somehow ended up underneath it, in lava... then I died.
DO YOU KNOW HOW HARD I TRY TO GO UNDER WATER EACH DAY? And you get it randomly? No fair xD! Anyways yea its a bug, a bug I will overuse to chill with the fish.
I had the exact same thing happen to me the other day.
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It happens sometimes when you respawn there I believe. I just used the "Ask Popo" skill to get out of there. You can stay down for as long as you want. It's just like moving around on the surface, but with fish.
By the way, I approve of the thread starter's in-game name. ;)