I bet that excavating underwater for archaeology is horrible. You don't want to damage anything, and anything you do is going to throw up a cloud of silt that will hide what you're working on.
Maybe you can pump the water elsewhere or something.
EDIT: Hah! They actually are doing that, show it in a video at the end of the page if you watch through it.
Is 850 feet deep enough to get the bends? I thought that was just dives deeper than a couple thousand feet.
I chose 850 because that's the average depth of the Adriadic Sea, though it has a maximum depth of 4050 feet, so if they are in the deeper parts, that would definitely be a concern. I would think they'd use a mini sub if it was though.