Hi miller,
When I first got my boat it was very tender...i.e. it rocked very easily.
I have since found out with the toilet tanks (2) empty, the boat rocked very easily. With the toilet tanks full or nearly so, the boat is much more stable.
Two years ago, I put an extra 1 tonne of steel ballast into the boat. Approx 600 kg. was placed almost exactly in the middle of the boat and the rest was split between underneath the seats in the lounge up at the pointed end.
Because one toilet usually gets used more, the ballast in the lounge can be transferred from one side to the other to counteract any list every few day.
This tonne of extra ballast increased the draft by about 3" at the most and with a cruising draft of 22" max it is no great problem with scraping the bottom.
When I added the ballast I was able to bring the boat onto a more even keel instead of dragging its bum by about 6" (stern trim) and now the max trim is 3" by the stern.
The moral of the story is that if you add weight low down, the boat is less tender....it works on the various ships I have been on for the past 35 years.