I would suggest using water as your ballast. In the worst case ,flooded boat ect, water will have neutral boyancy . Using sand or other heavier than water items will result in negative boyancy when submersed , helping the boat sink!

: Does anyone have experience on adding weight for stability to approximate a
: loaded boat for cammping when day paddling? I have read about water in
: containers and so forth but want to be sure the weight stays low, is
: distributed evenly for trim and won't tumble around when leaning or
: rolling. I have a Merganzer 18 hybrid as well as a Necky Looksha IV.