Many years ago I bought an old steel pontoon boat ( it's all I could afford) naturally the bottom had pin holes in it so I put a valve stem into each of the vent caps and would pump air into it, sorta like a submarine blowing it's ballast tanks.
It worked ok but the amount of water in the pontoons would change with the temperature so I kept a hand pump on the boat.
If you pulled the boat up on the beach it would let the pressurized air out but as long as it was in the water it worked.
