Abstract: The ullage space gases of a ship's ballast tanks (i) are depressurized to a pressure less than atmosphere—called an “underpressurized” condition—of most preferably about 14.7 PSIA or lower, simultaneously that (ii) they are inerted, preferably to <4% O2 and more preferably to <2% O2. Oxygen within the ballast water is rapidly, preferably within hours, substantially depleted, preferably to <1% PPM and more preferably to <0.6 PPM, by circulating these inerted ullage space gases through the ballast water as bubbles, removing oxygen from the ballast water more by outgassing than by diffusion. The rapidly de-oxygenated ballast water is preferably so maintained for the substantial portion of the ship's voyage, which is effective in killing most aerobic aquatic life within the ballast water.