As you say maybe the Money Pit was a diversion tactic. What if the story about the first 3 guys finding a small treasure were true. Most would have stopped looking then, right. But they kept digging and then as others came in to dig, hit the booby trap of water, which leads them to think we're on the right track or why do this. Mean while the real treasure is elsewhere on the island, and maybe just maybe Samuel Ball found it and that explains him being pretty well off for the times... and not from selling cabbage for 10 cent a head...