In any case, these ships really need to go into permanent graving docks where they are out of the water while also having the sides of the hull held in place by horizontal supports. They can't stay in the corroding environment forever. No matter what you do it will eventually dissolve the ship's hull.
Probably the cheapest way to do this is to coffer dam the vessels in situ, stabilize the hull with temporary shoring and then build the graving dock around them. Some of these ships are in such a bad way that this is really the only option as they are unmovable, as in they would break up under the stresses of being moved.