Is the contractor not insured?! That is one scary contract in my opinion. If he is stupid enough to hit the overhead wires, then why should you have to cover the cost? I would have to rethink this guy.
What are you putting in? An above ground pool? If so, what is the deepest that he will have to dig? They have a tool called a probe, it is basically a metal rod with a tee handle on the top, the tip has a pointed tip that is larger diameter than the rod, you could run around and probe for rocks, but not everyone has a probe. I used to install pipe, so finding things underground without ripping them out with a machine was a good thing. So I have one, you can get them at a plumbing supply house, I am not sure how much they are though as I got mine from the company I worked for.
I don't have any experience with pool contractors, but at the very least, they should guarantee their work! Maybe you should put it up yourself. I have been involved with two of them and did not find it daunting at all, but I have been raised around the construction industry.