It happened to me 20 years ago in FL, only it was a bail of weed. (and very soggy)
Another couple found 2 more and had called beach patrol.
Yea square grouper was everywhere years back, bait for public to unload somewhere else, same with kilos of cocaine, one guy found a kilo, and saw it had become wet, sipped the liquid and died many years ago... Odds are it was misthrown one boat to another, but he should be paranoid, could have some of ohbammas booted thugs looking for their lost kilo..
Seaweed I remember those days. Florida was crazy in the 70's/80's.
Kirbdog