why would they put water in the tank, makes no sense, u cant weld it up, water would piss out, so i'd say he put a bit of water in to say it was more full than it really was
One would have to be completely insane to weld a tank full of explosive gasoline fumes. Easiest way to get most of the fumes out is to fill tank with water. There will still be left some explosive fumes but space for expansion is now restricted(read: less drama when welding). Water is also safe choice due to high flash point (2 000 000 celcius or so) . Ventilating the tank so that there would be no fumes left is impossible.
There are two ways of approaching the "pissing the water out" problem. One is to compensate gravity around the pissing tank or to rotate the tank so that piss hole is in the uppermost part of the tank. You choose!