That seems like it would be really bad for energy efficiency: now you need batteries large enough for propulsion during the whole trip, plus extra for extracting water vapor.
Why not just take on some liquid water at the destination when you drop the cargo?
Because that's less flexible and there may not be water at the destination in the first place. That said, Flying Whales are trying to do exactly that, because ballast tech just isn't capable enough yet.
Why not just take on some liquid water at the destination when you drop the cargo?