Assuming that the MacBook isn't actually *in* the shower, but is on the other side of the room (that's what you meant, right?), you still run the risk of triggering the sensors and voiding the warranty. IIRC, there was a bit of a fuss a year or 2 ago about the iPhone water tags being overly sensitive, and people losing their warranties incorrectly.
Whether or not you'll actually cause damage is more debatable. If the room is visibly steamy, then quite possibly. If not, then you might get away with it. The question is, though, why risk it...?