Be very careful with dead weight. Water is neutrally buoyant when swamped. Lead, concrete, iron, sand, etc. will bring your boat to the bottom fast. Lead is usable, but keep it below about 350 lbs for the 20-22' boat range.
I do have sacks that are much tougher than most. They are made of the same material that some liferafts and inflatable boats are made of. The material is coated woven nylon and is very thick. Anything can be cut or punctured, but it would have to be something serious or deliberate to get through this material.
rider@boarditup.com FYI - I was a USCG Officer and was an Investigating Officer for many sinkings. My advice on dead weight has a bit of science and experience behind it.