I won't turn water on...with or without a waiver. Early on, against my better judgment, I let the buyer's agent do it in a vacant home. He had assured the buyer--who assured me-- that the listing agent "guaranteed" that it would be on. When it wasn't, I was asked by the listing agent to turn it on and I politely refused, citing liability issues. I allowed the buyer's agent to borrow a wrench from my bag and turn it on. I went inside to monitor for problems. Sure enough, a fitting in the crappy, exposed DIY CPVC clothes washer connections blew apart. Water shot straight up to the ceiling. Fortunately, no one asked me to pay for anything.
My standard procedure when booking the inspection is to ask whether ALL utilities are on if it's a vacant home. I've actually booked a lot of jobs by doing that simple thing. After I explain that I really don't want to do an inspection where I can't do a complete and thorough job the client figures out that I'm really looking out for their best interests, not trying to shorten the time I spend on a job. Seems that a lot of inspectors around here never ask about utilities. I hate to sound cynical (but I will) by suggesting that they like to find utilities off upon arriving. They can then point out that it's not their responsibility to have utilities turned on. They insert a quick disclaimer about being unable to test water related items (and/or gas) and finish early. They go home happy since they get paid in full while having spent less time on the job.

The client doesn't feel so happy. I prefer a happy client to a marginally shorter work day.