I labored in the tub but didn't deliver in it with my daughter. Our hospital had waterproof fetal monitoring strips, which I was required to use because my daughter was posterior and her heartrate was really erratic throughout labor. I pretty much labored on hands and knees in the tub. You just find the most comfortable position and go with that. As the PP said, water can slow down progression, so you want to use it sparingly early and get in when you really need the break. I was in and out of the tub for most of labor and then stayed in through transition when it was really really tough. So if your midwives can't monitor while you're in the tub, they shouldn't have a problem monitoring you periodically when you get out.
Beautiful baby girl born 8/13/2012. Little star baby lost at 10 weeks pregnant, 12/18/2013. Rainbow baby girl born 12/22/2014.