The short answer is that water won't cause any problem, and even salt
water probably won't (except corrosion).

When water bridges across a driven element or to the boom (not by any
means the same as the Earth), you've put a high value resistor and
capacitor across the driven element or from the element to the boom.
Pure water is a good insulator but acts also like a lossy capacitor at
RF; rain water on an antenna will have some dissolved minerals so that
reduces the shunt R. The feedpoint impedance of a beam is quite low, so
all you've done is put a high value of impedance across a low value. The
change is therefore very small. This holds for half wave dipoles,
quarter wave verticals, and most other common antennas, too.

A few antenna types have high impedances at the feedpoint, like an
electrically short whip, or a full wavelength dipole. Although I don't
know of any rigorous tests, I don't think even these are commonly
bothered much by water.

Roy Lewallen, W7EL

Dan Jacobson wrote:
Can one in theory still transmit if rain
water creates a bridge across the driven
element, or even also to 'ground'?
A DC short circuit but not a RF short circuit?
Or is it just salt water that is worrisome?