There's a misunderstanding here with regards to Science compared to Maths
Maths requires complete proof - it's one of the attractions of the discipline to people - if you prove something in maths it's good for all time - there never will be a right angle triangle on a plane where the squares of the two shorter sides don't add up to the square of the longest.
Sciences are somewhat different - you're dealing with the real world and so the definition of proof is somewhat different.
In Maths every triangle is similar by definition.
Every Helium atom is similar - not by definition but by experience
You might say to me 'Ah you haven't checked every Helium atom in existance there might be one that has three protons and two neutron and yet behaves like Helium'
That is why we have a definition of what constitutes scientific proof
You may recall last year when the Higgs Boson was discovered there was talk about 5 sigma
This is the definition of accepted proof - it's a statistical calculation and it equates to about 1 in 3.5 million chance
http://blogs.scientificamerican.com/observations/2012/07/17/five-sigmawhats-that/
If you want formal proof that dowsers cant do it you need to estimate the number of dowsers and 5 sigma will tell you how many you need to test to 'prove it'
Don't reckon your chances of getting a research grant for it though!