Whatever weight you use as ballast, consider filling the rest of the storage area with inflatable float bags or something similar to hold the weights "down" against the keel line. This is similar to your "loaded for camping" arrangement where dry bags with clothing etc. hold the heavy stuff in place so the load can't shift. Personally, I'd trust this more than Velcro (especially after the Velcro has gotten sand in it a few times).

Having a loose water jug rattling around in your 'yak while rolling can be quite an adventure - not only does the weight shift in weird ways, but it can also test the integrity of your hatch closures. Bounce an 8-10 lb water bottle on the inside of your hatch & see if anything stretches enough to leak (or worse, the hatch cover comes completely loose!)

Happy practicing!

Val