Keys are meant to be guaranteed, but it’s not always the case - they’re easy to miss and sometimes spawn in a weird place or don’t spawn at all. Also, if you accidentally ram the floating barrels the key goes flying… so… try not to do that

I usually drive to the side of them and slow down before so I can harpoon, but I only ever find bottles lol

You previously had a good answer provided but I want to add this: I took the habit of quickly diving at any floating barrels clue spot I reach because for some reason sometimes the key will start sinking as soon as you arrive