I agree the water bag ballast for trimming the load is the most efficient, especially for the Point Bennett single, which behaves well. However, the scaled up PB double, with a faster paddling speed (more bow wave pressure) and a higher paddler weight to floatation ratio (especially with more weight towards the ends) tends to weathercock and broach more, making a skeg on long crossings more helpful for it. I developed the skeg system mostly for the double.

:I still vote for your water-ballast bag as most efficient.