From The Doula Guide to Birth:
Warm water promotes relaxation by increasing circulation and lowering blood pressure, by stimulating the skin on your entire body (like a water "massage"), and in the case of baths, by providing buoyancy which takes strain off of the heavy uterus and belly. To reap the full effects, make your showers and baths in labor long, especially at the hospital, where there is plenty of hot water. For the most pain releif, it is recommended that you stay in for up to ninety minutes at a time!"
Ideally in a tub you want it deep enough that your entire belly is submerged but that can be tricky in a standard home or hospital time. Labor tubs can be rented (as inexpensively as $50) that are deep enough to allow this and big enough to let your partner throw on some swim trunks and hop in with you. (While I did not think I would want this option, when the time come I really wanted my husband's arms around me and I LOVED giving birth then pulling our baby up into my arms, with m husband's arms around us both.) If you aren't comfortable birthing in the tub then laboring in the time is still fantastic and I've done this at the hospital and birth center. Though in both cases the water made me relax so much that I dilated from 5 cms to almost full dilated/pushing very quickly and it was intense. In those cases I did feel more comfortable hopping out of the water, so do whatever feels best to YOU, of course. And it's up to you and your care provider if you deliver the placenta in the tub or not - I hopped out for that part.
I did not know until just now that when your uterus contracts it naturally leans forward so if you are laying back then it's fighting gravity. If you are laboring upright, leaning forward on a birth ball or on all fours then your uterus is already leaning forward and it may be less painful. That helps explain to me why I was naturally leaning like that in labor! And in the tub you can get on all fours or kneel and lean forward into the water for some relief.
Both hospitals in Denton have the option of laboring in tubs, depending on your care provider. Denton Regional has some in the rooms, though they are not deep and last I heard their policy is no water births. Denton Presby has a designated tub room (though they did admit it's mostly used as a storage closet!) and they also say no birthing in the tub, though I'm aware of exceptions being made if you ask a specific OB. 🙂