I know Swedish — to an extent — as it's the second official language in Finland.

Rarely used, so my vocabulary is absolute garbage and I'm also influenced by suomenruotsi, the Swedish that Swedish speaking Finns speak.

In Finnish, "kissa" is a cat. Just the lemma basic form for the word "cat".

I did remember how "havet" is the sea, but other than that my first impression was "oh how [adjective I can't recall], cat in the sea!"

That being said pissa is Finnish for "pee". Like the noun. So the pee of a cat is "kissan pissa".