LOL I do weigh myself just like that too. I turn the shower on, use the bathroom, and then hop on the scale! It's the only time I weigh myself ever!
I work at a medical supply store, and the fat monitors do really work. I would just make sure you get a good one, I'm not sure how well the cheaper ones work. Let me know if you need any suggestions on one. I've been thinking about getting one for myself too.