Just thought I'd chime in with something that should be blazingly obvious. Apparently after digging down so far, the hole flooded with sea water. Okay, so let's take it as a given that the "original" hole supposedly sunk to bury the treasure extends below sea level.
Even the best mining and excavating technology of the period when the hole was supposed to have been dug would have only been capable of reducing the level of the incoming water at best a few inches, maybe a foot. We're talking bucket chains or maybe a hand-driven water pump. And the pump is unlikely, because it would have had to be at or near the bottom of the hole because you can only suck water up so far before the difference in pressure causes it to turn to water vapor. You have to push water out of a deep hole, not pull it.
There is no practical way (based on my understanding of the technology of the period) for the shaft to have been driven below sea level by diggers.
I suppose that if there is some kind of natural subsidence phenomenon like sinkholes, it might be possible for an item buried at a higher level to be carried down to a lower level over time. Common sense tells me, though, that once you've hit seawater, the odds of finding anything deliberately buried are nil.
Beanbag