You can pack the pipe with white bread to stop the water from coming out while soldering. They also sell a dissolvable foam type product that does the same thing as bread.
I keep a length of clear tubing in my soldering toolbox that fits inside half inch copper pipe. I either syphon the water out of the pipe, or I put the tubing as far in as I can, then plug the end of the tubing with my finger and pull the water out in the tubing.
If I'm lucky, they house I'm working on has an outside spicket that's lower than the water level and I can drain it that way.