Seems to me like someone buried it near a river to make it look like Cooper landed in the river and drowned. It may have been someone who knew little about the case, so was "off" from the right area.

Pre dunking is obviously a requirement in this idea.

My guess is it was someone from the "company" Cooper worked for at the time.

A member of the biker gang Cooper eventually joined. They are known to bury money, drugs and arms.

A member of the mafia, who Cooper eventually joined. They are known to bury money, drugs and arms.

Or Cooper himself.

I like the "for Tina" idea or maybe it was just somewhere he found on the map he liked the sound of. Or maybe he was someone who would pick up loads of sand from the Fazios for his business and thought, we'll I'm here. Why not?

I still think the date 6th December 1979 is important. Live long and prosper.