You will not find any hospital in the Metro DC area that offers water births.
There are a couple that have tubs that you can labor in *if* 1. the room with the tub is available when you arrive at the hospital, 2. you have signed orders from your OB/MW allowing you to labor in the tub, 3. you have absolutely no issues with your pregnancy or labor that they think might cause a liability issue if they allow you to labor in the tub (such as non-reassuring fetal heart tones while you are being monitored, slightly elevated blood pressure, etc) You will have to get out of the tub to push and birth.
Of the area in-hospital birth options, I would suggest you look into Loudoun Community Midwives first. It is a bit of a drive from Alexandria/Arlington, but worth it. Physicians and Midwives in Alexandria used to be very supportive, but their practice has grown increasingly restrictive over the years, and I find it difficult to recommend them now. I had a client who had a very abusive experience with them, simply because she was a 39 year old primipara. Dr. Tchabo at Virginia Hospital Center (Arlington) is good, but not always on call. He is supportive of natural birth, but has gotten a lot of pressure over recent years to "toe the line". Stay far away from Fairfax Hospital if you can, unless you have a super-high-risk condition that requires immediate medical attention during/after the birth.
If your situation changes and you can use a birthing center, the Family Health and Birth Center in DC allows waterbirths, and I think the NOVA Natural Birth Center in Chantilly does, too.