You don't necessarily have to "tell her off". Even if she's with her girlfriend 100% of the time, just politely mentioning it isn't antagonistic. I've always felt like text messages were just as passive aggressive as notes, and the issue with a "can we talk alone?" text is that it might make her feel anxious or defensive while she wonders what you want to talk about all day.
Just be like "Hey, I was thinking we could cut back on the length of showers since the water bill's been a little high lately". That's not antagonistic, it's not passive aggressive or confrontational, and it's not something that ought to embarrass her in front of her girlfriend. She'll probably catch the gist of it.
That way she can't claim you confronted her in front of her girlfriend, or that you did something passive-aggressive like sending a text/leaving a note, and if she gets argumentative about it THEN you can suggest she chill out and you discuss it later in private. If she doesn't catch a clue, then next time she takes a mega-shower you can send a text about setting up a time to talk about it in-depth. Plus that way she can't complain that it came out of the blue, either.