I have used a water level when I was building a long retaining wall in my yard. Not a fancy one, just a long plastic tube with water in it. Now way for me to measure any diff.

As far as using something to check for foundation level, I would use something else. Something similar to a survey tool probably.

That said, I don't think I would offer this as a service unless I was qualified to do so, such as an engineer or surveyor.

That said, I have been on many jobs (in CA) where the foundation contractor was using a laser transit to set his forms. I'm sure he probably wasn't an engineer - and I'm not sure that ONLY an engineer would be qualified to use those tools. Checking a foundation for level is not exactly rocket science, and probably doesn't require advanced mathmatics or calculations to determine IF it's level, or for that matter, how much OUT of level it might be.

As far as "exceeding" SOP from any organization is something that just about every inspector does on a daily basis. In fact, the ASHI SOP actually allows inspectors to include other inspection services. SOP 2.3. "These SOP are not intended to limit inspectors from:
A. including other inspection services, systems or components in addition to those required by these SOP".

Just my take - your mileage may differ
JF