I’ve been using a water torch for a number of years now and, although
I also have a Sievert propane torch and a Microflame (rather like a
Little Torch) oxy/propane unit, I use the water torch for almost all
my soldering and annealing work.
I don’t use special flux; I use the same stuff I use with the other
torches. I use a liquid called Auflux (slightly luminous green watery
liquid) for precious metal, and EasyFlo for ferrous metals.
The water torch gives a narrow, intensely hot flame, and I suspect
you are directing it right at the joint rather than waving it around
it. You are thus using the flame to melt the solder directly rather
than using it to heat the area around the joint so that the joint
itself melts the solder.
That’s all I can think of because, like I said earlier, I don’t use
anything special with mine.
Regards, Gary Wooding