Oh dang
I would say plain cool water. Not so cold that it'll hurt your hands, but cold enough that it would keep the hash hard, you know?
Yeah I'd say chop it into 'worms'. Put it in a fridge or freezer beforehand to make the cutting easier. Pop in in a coffee filter in a bowland run some water over it. So you'll have a coffee filter within a bowl full of water. I say this just to minimize chances of losing anything.
Now I said cold water becasuse you should cut it into 'worms' rather than an assortment of chunks, you know?
Because you've got worms, you can put your fingers in there, while it's submerged, and roll the worms back and forth so they get thinner, then fold them back into a ball and roll out again. The heat from your fingers should be enough to make it malleable, while the water will keep it cold enough that it doesn't stick to your fingers very much, if at all. Razors can fix that anyway.
Do that once, roll it all back together and then repeat with more clean water a second time. That should leave you with pretty clean stuff. It's a lot of work but you were right on your first guess, that water would be the way to go.
Maybe try distilled just to be sure?