Driving one with a cup of water:
I use a 32oz cup full of water. I start the ground rod by simply pushing it as far as I can get it and still withdraw it. Withdraw the rod, fill the hole with water, replace the rod, and shove the rod down HARD, as far as it will go and still pull out partially. Pull up on the rod two feet or so, but not completely out this time. And repeat. Trickle water down the rod to wash off the resistance of the mud on the rod each time you pull the rod back up, enough to wash off the rod mostly, but no more is needed. The water also will run down into the ground along the rod, and as you shove the rod down again, the water provides hydraulic pressure by being compressed in the hole by the rod, and this shoves dirt out if the way.
If you've ever watched an older well drilling rig, they use the same method to drive a casing to some pretty good depths.
Of course, this works best in morain soils, since really sandy soils won't contain the water in the pounded hole long enough to create the pressure needed, and rocky soils will cause the same problems AND the rocks themself block the rod. Clay soils can be worked if you find some sand to help loosen the grip of the wet clay, but it's a job.
Now, there were a couple of inspectors who didn't like us "drilling" with water, but none could ever cite a Code violation. If you wait a while for the ground to drain, you can't pull the rod back out, so the one concern of a "loose ground" seems groundless!