This is my favorite show on t.v., I could talk about it for hours.
I like how the story is really all the people who have been involved with this over the years, not really the treasure.
We don't really know if the early 1800's expedition (or the late 1700's) actually happened, there's newspaper articles from the 1850's saying that the people who did the one in the early 1800's were involved, but they could have been creating the hoax at that point (1850's expedition). This is the most cynical view in my opinion, we essentially know everything from 1850 on happened.
I basically believe most of the story (even the late 1700's origin part), but think whatever was down there is long gone, possibly long gone by the late 1700's.
My biggest theory is that perhaps there's some kind of way that all the stuff they find really deep is somehow being washed in through the system of caves below the island.
So for instance, maybe a ship wrecked 1000 yards away, and when they find the bones, the old wood that deep, it's something that somehow washed under the island through a cave or sinkhole or something.
The 'coin in the swamp' they found the first season I'd bet money the production crew planted. Probably bought it off ebay.
Another thing they don't really talk about is the gentleman in the 60's dug a BIG hole, people. Some people claim he went down 200 feet and 200 feet wide. If he did then that explains everything in the hole...
There's a small chance the hole funneled and he 'missed' the sweet spot at the bottom.
So i'm at like, 50% chance something was there that's now gone, 40% chance it was a hoax the whole time, 10% chance something is still there.