Sound pretty silly to me.
Yes, they carry a huge amount of water, but the problem are being able to get a lot of it out quickly enough, making sure that the centre of gravity doesn't move around too much, finding a runway that's long enough wide enough and stroung enough to take the big plane, and keeping the brakes cool enough for a turn around of less than about 45 mins or so.
747's also don't turn very well, don't climb all that well, etc.
I cant' see it being terribly useful other than one big-ish drop, then a long wait for another, and so on.