Water=8.35 lbs/gallon. A smaller tub holds 50 gallons. You are talking 550lbs of weight per person taking a bath, and that is in a very un-luxurious tub. Not to eco friendly or economically realistic to burn all that fuel, at the very least for takeoff and climb and part of cruise before dumping it. The 400 gallon hot tub would weigh (just the water, not the pump or housing) 3,340 lbs.

Think what SPD or overnight air would cost on 500 lbs of shipping.

I read a figure (don't know if it's true) that the new (combined) UA uses more fuel than is produced worldwide in a day, each year. Those carbon footprint offset/credit people would have a field day with a commercial airline doing this.