First idea:
on-board super high powered de-humidifier. you could dump your ballast just before a climb, and then have the dehumidifier pull water out of the air and replenish water ballast for the next cruise during the climb.

or maybe that's too insanely fast to condense 30 gallons of water out of thin air.

I worked on an experimental "Water From Air" project a few years ago designed for arid climates (think Afghanistan.) The Mil-Spec called for something that could produce 500 gallons of potable water in 24 hours on 100 gallons of diesel (or jet fuel, JP4, Jet-A etc.) Other specs defined the climate as around 90 degrees F and less than 20% relative humidity.

The machine met the specs, and worked pretty well. It was also eight feet high, 16 feet long and eight feet wide. Weighed around 10,000 lbs. if I remember correctly. Ran on a CAT Diesel generator and made enough noise to wake the dead.
It was designed to fit through a C-130 rear cargo hatch and ramp. It did, with an inch or so of clearance.

So, that's 50 gallons in 2.4 hours on 10 gallons of fuel. Want to buy it? I can probably get you a deal, if the Army hasn't destroyed it yet.