You cannot effectively solder a pipe that contains water. You have to either drain out the water or block the water from entering the pipe. In some cases you MAY be able to use a really hot torch (acetylene/swirl combustion) to get the fitting hot enough to seal the joints, but that is an iffy procedure, and it only works for pipes that have “some” water in them but not a “lot.”
You should always be able to do the job right, with a little or a lot of effort and common sense. In such cases as yours some people have resorted to trying to tape the joint or seal it with various glues or clamps… My advice is do the job right.