[wrycoder]

The resistance of slightly salty bathwater is much lower than air. While baths are not often plumbed with metal pipes these days, one can still expect some current flow to ground. It only takes milliamps of stray current to trip a GFCI.

[mindslight]

Milliamps sounds like a tiny amount in the context of home power, but 5mA at 120V is still only 24kOhm. This is actually pretty low to just assume away. I'd agree there could be some water left over in the drain pipe from the last bath, splashes making a path to the spigot, or something like that. I just wouldn't count on it.