Here's a thought: Maybe an open water diver found the clothespin and figured it might belong to whomever owned the reel. Since he wouldn't be around when you returned, he stuck it on your line where you'd be sure to see it.
Whoever said money can't buy love never bought a puppy.
While I would want to hear what the official answer is, this is what I thought too. I have had someone put a line arrow on my wreck line when I was in. Taking into account the circumstances it could not have been placed as meaning anything else but "this must be yours" or as a joke. It was too hard a joke to get* so I picked it up. In that case the arrow was also left very close to tie off, outside the wreck, so I just assumed it had no significance other than being picked up from the bottom.
*Other than the initials being DB which always makes me think it's Mr Dingle Berry and giggle
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