When I was a teenager, a neighbor in his 50's showed me how he "witched" water and had me try it in an area where he knew there was a deep buried water line and I found it. He only used a fresh-cut forked branch from a peach tree. He said that some other tree species worked also(species that like to grow where there is lots of ground water like willow), but that peach always worked best for him. The branch should be cut just prior to use, a dried out witching stick won't work. If you have to cut one a day or two ahead of time, then keep it cool and damp in between a couple of damp towels inside an ice chest.Once the stick bent down to a spot, he would step back a couple of steps, get a new grip on the ends and step forward. He would repeat this until the branch no longer responded to the spot. He counted the number of times the branch bent down and said that the water depth would be 4 ft x the number of times it bent down. This is not a vague dip in the branch. You hold it tight and the branch will turn forcefully and twist the bark off in your hands. That part worked for me as well and I told him how deep the line was and he said I got that right too. I haven't had any reason to practice water witching and haven't tried it in years, but I think it works if you know what you are doing.
On the other hand.... I know a guy that drills water wells for a living and he doesn't really believe in water witching. He said there is ground water in most places -- it just a question of how deep and how much-- so the most important reason for choosing a well site he says should just be a matter of convenience in regards to where you need to use it.
As far as metal dowsing rods-- someone showed me what they would do when I was a kid also. Small diameter smooth brass welding rods, those used for acetylene welding, is what I've seen used and what I have experimented with. Mild steel rod will work also.Bent in an L with about 6" for the short handle and the other leg about 24"long. Make two and hold one in each hand. Start off with the rods parallel to each other and about 8" apart and hold them level. Walk around with the rods pointing straight ahead. They will repsond to any metal in the ground.If it is off to the side, the rods will turn that direction. Once you start to zero in, the rods will cross when you are directly over the metal object --X marks the spot. If the metal object you are approaching is very large it may separate the rods and kick them away from each other (sort of like an overload mode for the cheap detectorist). Holding the rods further apart may help then. They respond to very tiny pieces of metal-- like even a piece of bird shot. My experience/opinion is they're like an inexpensive metal detector that doesn't have any discrimination ability or depth meter. You won't know the depth until you dig it.They will respond to any type of metal and very tiny pieces of metal. They will even cross on pieces of old glass that contains some metal in it, you know, the old clear glass that has just a tint of purple to it? I think it had some maganese in it?
So my opinion is that I might use them if they were all I had, but if I got a metal detector, it is going to tell me a whole lot more about what's in the ground.
As you're probably aware, you can buy manufactured dowsing rods that have fancier handles of small pipe or ceramic that the L of the wire/rod sits in so it can turn more freely (but if you hold the plain rods loosely in your curled fingers they can turn pretty freely as it is) and some I think supposedly have crystals in the handles and/or holders located on the rod tips,etc. that supposedly make them respond to different metals better than others. So yes, dowsing rods will find metal, but I don't think they will tell you any of the what kind, what size, how deep info that we seek from our machines. Of course, someone that it is really into dowsing may have a different opinion.
As far as map dowsing goes, I think that is just bogus. You have to get the rods in the immediate vicinity of the metal objects before they respond, just like you have to get the coil of your metal detector over the target before you find it.