I’m a little rusty with the plot details, so I can’t really answer your questions. But I also wondered about those puzzle pieces. It really took all those pieces before they could figure out the location? Wouldn’t just the piece with the X be enough? And speaking of that, the X isn’t exactly specific itself. Wouldn’t the loot still be kind of hard to find?
The X piece by itself didn't have enough of the surroundings to see that it was a spot in the water, next to a pier in the Marina Del Rey Channel. They probably really just needed the "X" piece and a few around it. Most of the bottom pieces were just water.
I think it was fairly easy for them to send a diver down and find the loot.