Before revision: While it seems counter-intuitive that placing water in the hull (which is, after all, close to the same density as the water outside the hullfresh vs salt water) would add any stability, adding water ballast below the vertical center of gravity increases stability.
After revision: While it may seemthe water serves to displace air from the bottom of the hull; counter-intuitive that placing water in the hull (which is, after all, close to the same density as the water outside the hullfresh vs salt water) would add any stability, adding water ballast below the vertical center of gravity increases stability.