The jobs are clearly very different but what we are discussing is doing direct compensation for the risk that third-party people are exposed to in the case of a failure. The theory is that by giving the operator higher pay you reduce the risk to third-parties, in which case water treatment failure is one of the top risks that can effect a very high number of people. A midwife that makes a erroneous decision kills one or two people, a erroneous decision in the water system of a city can kill and serious harm thousands if not hundreds of thousands. Similar is true for nuclear plants but those tend to be already well compensated.

If we ignore the risk aspect to third-party which parent and grand parent comment brought up, and instead focus on pressure, I can think of a few ones which might be worse than midwife and with lower pay. Highway road workers would be one. Having trucks going by in almost highway speed just meters next to you while you are focusing on the work is the kind of hell on earth that I would not want to expose myself to, and it doesn't even pay well.

Other high pressure jobs on the top of my head would be rescue swimmer, as the profession require careful psychoanalytical pressure testing to enter the training program. It also takes a special kind of person to work as a explosive chemist or at the bomb squad. I would also mention the military as an natural example of high pressure job. Each three has widely different degree of compensation compared to each other and even within the professions itself, and compared to midwifery it is all over the place.

There is no clean formula for X(Pressure) * Y(Third-party risk) = Z(Pay), and implementing one would change the compensation for a wast number of professions. I strongly doubt that midwifery would be the one that would see the biggest effect, but I would love to see a study exploring it.