Pissing in the ocean would make more sense but only as a saying - not the action itself, if say you were trying to raise the sea-level. I imagine it might be used as a lame excuse for not contributing to a charity that tries to combat mass poverty etc.Hi Caroline.
I've never heard that one before, although my guess would be the same as yours. I imagine someone peeing in a lake or some large body of water where it will only get diluted anyway.
It's not very common anyway, at least not in my circles