I know that I am having similar issues, with no water damage.  It all started after the 5.0 update.  I have had to turn my WiFi off at this point, since the constant on/off gives me a very unreliable connection.  I have wiped my cache as well as a factory reset, and no improvement.

I would guess a possible bad/incompatible driver for the S5.  Hopefully Verizon has 5.0.1 or 5.0.2 in the works, or maybe a hotfix coming soon.  I'm going to end up eating up a ton of my data plan if I can't use WiFi.