FYI, literally no-one calls them that in the UK either. While you may sometimes see WC on, usually older, signage or maps, it's at least as common to see "toilets". It's invariably those letters and never written out in full. I think if you asked people what WC stands for, write a few simply wouldn't know.
If you're the type who considers the word toilet vulgar, you'd likely say "lavatory" instead. If you're trying to be actually polite rather than just signal poshness, you'd make a discrete exit without stating your exact excretory intention.
It may also surprise you to learn that water closets are called restrooms in other countries that speak English.