Inflatable ballast for dock (permanent)

We have a section of dock that needs additional floatation as
it has lost freeboard. I need something that can be inserted
under the dock as it is and then inflated. I know there are
those vessel salvage bags. I’m not sure how sealed those ate
as I don’t want to have to refill it often.

anyone have any recommendations?

 

I know of no flexible, inflatable bag intended for permenant use. They are all temporary. Diver lift bags and salvage floatation bags are all temporary and quite expensive to use for permenant floatation.

Most often people use barrels either plastic or steel. Both bungs/caps are removed from the lid. Hold is sideways in the water until it's mostly full of water. Then put it in position under the dock and orient the openings at noon and 6:00 (one above the other). Cap the top one. Then shove an air hose in the bottom and fill the barrel with air. When air comes steadily out the bottom hole remove the air hose and cap it.

I put plastic covered foam blocks under my dock. Instead of sinking the floats I lifted the dock. With old work boats (partially sink them, tie to dock, pump out water) and barrels pumped with air I was able to raise the dock enough to get the permenant, plastic covered foam floats into position.