Easy enough to drill granite. Rent a compressor, air drill, drilling iron. I've got two or three of the carbide-tipped bits that thread onto the iron...used, but resharpened. (I've drilled many a hole in my callow youth, using a tapered wedge driven by an air hammer / paving breaker, in order to break granite boulders in a basement that I was digging out.)
It's possible any of the larger electric hammer drills now available would do the deed. My Bosch, bought at The Despot, will go through 20+" of the local Wissahickon shist (of which our house is built) in fairly short order.
I think the tougher problem would be the attachment. It would be the item, that as it wore or corroded, you could never change. The shackles and chain would be renewable, either by replacement or addition.
Perhaps you could build a 'sacrificial' skiff with cleverly attached flotation, whilst at your dock, cast a reinforced concrete 'footing' in it using epoxy-coated rebar (like you see on bridge projects). Tow it out to the site and sink it.