I honestly do not believe anyone knows whether it is harmful or not. I think there are a lot of guesses, but that is it.
When I see something like this in the front of the Raymarine users guide, I would err on the safe side and heed their advice. I imagine the engineers have seen some pretty freaky stuff while testing these products. Ultimately, radomes emit a signal at a 25 degree angle, so 12.5 degrees below horizontal centerline. If you have that from stern to bow plus factoring the height of a tall person, than all is well.
From Raymarine User Guide, Page 5:
Radio Frequency Radiation Hazard
The radar antenna emits electromagnetic radio frequency
(RF) energy which can be harmful particularly to your eyes.
DO NOT look at the antenna at close range.
It is important that the radar is turned off whenever
personnel are required to come close to the scanner
assembly or associated equipment. It is recommended that
the radar scanner is mounted out of range of personnel
(above head height).
Distances from the face of the radar at which RF radiation