A fresh water well. There have been numerous dug in search of treasure around the Money Pit. Some to depths of over 140 feet and filled with Fresh Water not Salt Water. Maybe all that Vaughn, McGinnis, Smith and Ball found was a well dug in the early 1730's by the British Army for fresh water for their men that were stationed there from 1730 to 1738. It could also have been a shaft for mining of minerals as Nova Scotia as all kinds of minerals and mines even gold mines along Gold River.
and we suddenly take that info as fact from WAY back when and we don't take other stuff as fact from way back when.. Just saying. You can't pick and choose what is fact to fit your narrative... Not saying it isn't possible as I don't know anything about drilling for fresh water or drilling for it on a small island, but I wouldn't think that most people would be digging for fresh water at over 100' on an island that is only 35'-45' or so above sea level... BUT even if they did it would just be a pit, no need to brace it all off starting at the 80' level and down and not all the way to the top... let alone tunnel off once that far down..