This question might be answered better by LEOs I have had several people tell me getting water on a bullet resistant/proof vest weakens the ballistic fibers. Is this true? What about sweat and rain?

My info is a little old, but here it goes –
Kevlar’s strength is degraded by being wet.   That’s why the ballistic panels in the older vests (the only ones I am familiar with) were in a waterproof envelope.   I’ve heard that some of the newer fibers do not weaken nearly as much with moisture.

A good source of information is at www.secondchance.com. Richard Davis the founder of Second Chance invented soft body armor. He has been of the feeling that water does not degrade performance of a vest enough to justify the waterproofing. The waterproofing makes them hotter, which makes them less comfortable. In the past, this was somewhat true. But todays fibers are much lighter, the carriers tend to breath, so it is not so much of a concern. NIJ I belive requires vests to be waterproofed to help keep them from being degraded. Of course there has been a long standing argument from NIJ and the manufacturers over what was really necessary in a vest. I tend to agree with the manufacturers belief that a comfortable, slightly less protective vest will more likely be worn than a very protective, but uncomfortable vest. This is pretty much the truth here in sweaty Florida.  

YOW! You guys really wear a vest in Florida!  Man, I was on vacation in Miami, and I could hardly stand it just wearing my regular clothing.  It was so humid.

We wore vests on certain posts when I was in Panama in '88 and '89. The vest would be soaking wet when we took them off. We would hose them off to keep them from stinking then hang them up to dry for 4 hours until the next guard change... Or you could get lucky and get a post where you wore your flak jacket instead of the vest.(Talk about hot)
Steve