[poli]

I wouldn't worry much about the water. Whether or not you are very careful about thing tap water will reach your system in the form or ice cubes, salads, showers, etc. The water doesn't make me sick. I don't drink copious amounts and always buy a big bottle of purified or spring water at a convenience store as many Mexicans do. If you worry, get a prescription for Cipro or cefalofloxin, but use it only if you get sick and not as a prophylactic dose.
Also, drugstores in Mexico stock this probably.
Mexico City is very crowded but definitely a great place to visit, not just because it is culturally very rich, but because it can be used as a home base for many easily-reached cities and cultural sites nearby. Intercity bus service is very efficient and inexpensive, and buses run frequently to late hours in many cases. If you shop around, rooms at good four star hotels can be found for about seventy dollars a night breakfast included which is really inexpensive compared to USA or Europe. I don't see Mexico City as a particularly relaxing place, although is some parts it is. It's a challenge full of good surprises if you are willing to explore.