Treasure chest on the shipwreck lot

Heya :)

There is a treasure chest hidden behind a wall in the shipwrecks basement and it has a greyed out option to open it.
Until now I didn't find out how to open it, but if I hover it with the cursor it says, it needs a key to open. Does someone know, how to open it or how to get the key?
I already tried treasure diving in hope to find the key, combing sand and the cave where you can find the mermadic kelp, but no key so far...
Has someone been able to find something and likes to share?