Unless all the boats are moored using very short lines with no catenary curve the dock would have to sink a very long way before the lines tightened up enough to give support. It’s hard to see how the roof wouldn’t contact the tops of at least some of the boats in that case. And if instead it did happen as they plan, what if you are only side tied? Do you capsize or drag one side down to the water level and thus sink? Or what if you backed in and have short stern lines and long bow lines, will the transom be pulled below the water? Or if you have the shortest lines on the dock and your boat ends up trying to support the entire roof, do you then sink first?
This sounds like such a bad idea in so many ways.