Things growing in the water would be an issue. And highly treated water will damage your skin. There'd also be a chance of drowning every night. You could mitigate with a dry suit but that kinda ruins the point.
Besides all the potential medications you'd wake up panicked, in water (likely swallow more), and may in your panick fail to get out of whatever tub setup you have. It isn't common but people do drown in bathtubs (not just infants who've been left alone).