Wouldn’t it have to be in really cold water for the rum to be preserved?
Does anyone remember a movie about a ship full of whiskey running aground on one of the Western Scottish Outer Hebrides?
If I remember right the locals ended up with it all.
I would think that if the water is deep enough the temperature could remain 60 F or lower ... the rum may then have continued to age if in sealed tight bottles. But ... if the rum is in oak barrels I’m sure they would have rotted after all of this time.