I have dreamed about this over and over and over again. Only, I cannot take my own dogs to the beach, due to local laws. But, they enjoy digging. And, they would be great for digging up finds.
Also, when I was younger I dug the holes for a fellow who located metal pipes underground in chemical plants, during demolition. It was crazy. He would throw a stake down and say "Dig here. I will come back in a few hours." To my utter disbelief, I would hit deeply buried pipes every single time. I tried to use the method he used, and I could not do it. The rods would swing wildly in my hands. But, in his hands, the rods would cross the exact moment, he stepped over a pipe. Given that the chemical plant has massive metal machinery, and bins, everywhere, why would the rods swing when the fellow stepped directly over their underground location?? I don't know. At best, my guess is that the magnetic field interferes with the magnetic field of the human body. But why below the body?? Why don't large metal objects to the side of one's body affect the dowser the same way?? I don't know. I only dug the holes.
What I find remarkable is that today, in 2017, not much seems to be known about this subject. Yet, I believe even the Spanish used a willow branch to dowse for ore, centuries ago. And, it appears that they were successful at it.
Back to the dogs - I would really like to see whether people can train their dogs to do more of the finding, and digging. Or, can dogs even be trained to detect gold? Has anyone tried?
As for digging, it might be worth considering using the same age old tools that were used for centuries, by diggers - antler tipped tools. If antler digging tools are used, I would be willing to bet that we would see far less scratched coins, and/or chipped artifacts. Just my two cents.