[Gustav Schiebert]

The wavelengths would not be harmful to humans - microwaves are technically only found between 1 GHz to 100 GHz, whereas the FuMO's on a U-Boat were never greater than 82 MHz.

Even within those 'microwave' brackets, the power has to be very high to cause any health effects - and even then, the effects are short-term (ie burns) rather than any long-term effects associated with ionizing radiation, which is I suspect what you were after?

So radio waves like these are harmless.

The issue with that is, of course, that due to the casualty rates in the U-Boat force in World War 2, the number of survivors (about ten thousand) would not be large enough to observe any statistically valid health patterns in later life - so if for some freak reason contrary to medical knowledge it did cause any problems, we would almost certainly never know about it.