My boat was under water the first time I saw it.
I was a part of the salvage team and this is what we tried. (note I said "tried")
Attempt #1: Cheap float bags (water beds)
stuff the bags in the cabin, fill. The result was the bags provide so much lift that they found their way to the companion way hatch and blew out the lip that holds the slide in place.
Attempt #2: Tie plastic drums and fill (55 Gal)
The drums didn't want to stay down and there is no tie point on the round drum.
Attempt #3: leave lid off drum, poke hole in lip.
Worked much better, no lid means that swimming it down there was no issue, a bowline through the hole in the lip of the barrel to a strong point on the boat. Repeat for each stanchion, cleat and chainplate. Fill a little air from the scuba tank to keep it upright. Then someone passes down the compressor hose and you start filing. This still didn't break the suction.
Resolution: At low tide, Tie many lines from the docks around the boat down to the strongest points on the hull. (A sling under would be great but the whole keel was sunk in the muck). Let the tide come up. Here is the key point>>> at high tide, tie the boat to the pilings and let the tide drop. Now the barrels can help out and your good to go. Pump out the cabin really quick and up she floats.
Now this is assuming that you are a: near docks. and b: in Maine where we have 12' tides.
See my site for some of the salvage pics if you want.
~Sunk, Salvaged, Refitted, Sailing!