I'd suggest looking at the section on Adults and Microwave ovens. Over many cases of people being exposed to an open and operating microwave oven the only injuries were burns close to the surface and peripheral neuropathy from placing their hands inside the oven getting a substantial portion of the total power on a small area. The more serious injuries on that page were a result of much more powerful and more intense microwave sources at different frequency bands. If you stick your head into a high powered waveguide, you're going to have a bad time.

In fairness I was surprised to learn that nerve cells absorb a fair bit more than skin and fat leading to quicker nerve damage. All things considered though, I stand by my statements, hot water at a sink presents a much bigger risk given the frequency of exposure and the risk of burns.