I can tell you from personal experience that water is a lifesaver in labor. A birth/labor tub or a shower both work really well! For me, the shower worked significantly better than the tub, but I hear that's common in first labors. I just set the shower head to the massage setting and got the water to just the right temperature, and I was in heaven compared to not being in the water.
It helps the physical pain as well as being very relaxing. As someone who's struggled with anxiety and panic attacks, I learned years and years ago how relaxing water is and often used it to get rid of my panic attacks, and that's where I got the idea to get in the shower in the first place! I didn't even know it was a common method of pain relief in labor, I just knew I needed some help grounding myself and focusing, and if it helped me ground myself during a panic attack surely it would help me ground myself in labor!
I believe the birth pool or labor pool helps so much during labor partly because of the weightlessness aspect. In the water, you really do feel weightless, and it's much easier to move. That means that in a birth/labor tub it's much easier to change positions and find one that feels comfortable for you. Aside from that, the warm water feels nice and is very soothing and relaxing. It is said that laboring in the water makes for a shorter labor, which is also less painful. It is also said that laboring and birthing in the water usually results in fewer complications and less intervention!
Using water as pain relief during labor is also extremely effective when it comes to back labor. Trust me on that one! Because it helps so much to make you relax, it also helps you store/save energy for the harder parts of labor (hard labor, pushing, etc.).
Oftentimes in hospitals you will not be allowed to use a labor/birth pool until your cervix has dilated to 5cm. The reason for this is that is when you are considered in hard, active labor - and early labor can last for days. A birth pool should be emptied, disinfected, and refilled every 24 hours during labor. After you hit that 5cm mark, you're not likely to be in labor so long, and therefore they won't need to empty and disinfect it.
Another reason for this rule is that they don't want the water to slow your labor or contractions, or lead to other interventions. Forgive me here, but I don't buy that one.
Personally, I think that rule is in place mainly to make their jobs easier, but there's a way around it!
Just because you can't use the pool/tub until you hit 5cm doesn't mean you can't still use water as pain relief in early labor. There's no rule about dilation for using the shower, as it's constantly running so you're not sitting in the same water for hours. Like I said earlier - showers are heaven, especially for first time moms!
Another great thing about laboring in the water is that your partner or child/children can join you. In most cases, your midwife or birth doula will not get into the birth tub with you, but your hubby (or wife!) certainly can! It's even alright for your child to be in there with you - it's all about what you want and what makes you happy and comfortable.
You can drink while in the birth pool at home, at a birthing center, and even in many hospitals - and it's a great idea to. Be it water, a sports drink (like Gatorade or Powerade or this awesome, natural, homemade sports drink), or juice - it's good to make up for the lost electrolytes because of all the hard work and sweating you're doing!
If you're at home, you will be allowed to eat in the birth pool, but in the hospital you will not be - following standard hospital procedure, unfortunately.
The great thing about laboring and giving birth in a birth pool/tub is that it's great pain relief and there are no adverse risks or effects to mama and baby!