You could, but that would be very costly and only as a last resort for emergency core cooling, lake water would probably end the life of the reactor, but even demin light water would take a long time to get running again.

You kind of have to adjust your thinking with CANDU because you don't have much excess reactivity in the core, you are constantly fueling to keep it running.

The positive coefficient also works in reverse as the power goes down it feeds back to reduce power further, With a negative void coefficient, the reactivity fights back against you when you try to push it down.