I talked to somebody who knows more about water bills than I did and I'd like to share what I learned.
Here are the five different charges again and some comments on each:
Cargos for agua. This fee has a fixed part that you have to pay whether you use water or not. The lowest fee here is $10.60/month when your place is serviced with a 1/2" or 5/8" water line. Do you use more than the included amount (10 m3 in the lowest basic rate) you pay extra per m3.
Cargos por Alcantarillado Rey is right, this is the fee for sewer. It's based on the diameter of the incoming water line: the bigger the diameter the more you pay. This is a fixed fee, too. No matter if you even are in the house, you have to pay.
Cargos fijo Seems to be what I wrote before, a fee that has to be paid if the water is not AAA provided.
CCAR Fixed "extra" fee, also based upon the size of the incoming line. You have to pay this whether you use water or not.
Cargo Especial Like the CCAR fee, fixed, based upon the diameter of your incoming water line, always due water usage or not.
If I apply the above to my bill, I will have to pay $10.60 (cargos de agua), $6.50 (CCAR) and $ 2.00 (Cargo Especial) for a total of $19.10/month, water usage or not. The only variable part is what I use over 10 m3 (last month 3.5 m3 at $1.25 = $ 4.38)
If you have a bill that's (a lot) higher than mine, check two things: the diameter of the incoming line and the actual usage on the water meter. For a normal house the incoming line doesn't need to be bigger than 5/8" keeping you in the lowest scale. Bigger lines may be needed if you have a pool, many bathrooms, an automatic watering system for the yard etc. I don't know (yet) if it's possible to get a smaller line if you would want that. A licensed plumber or the water company should have the answer to this one.
If you use/pay a lot it might be a good plan to think about using collected rain water for watering the yard and flushing toilets . Of course the latter involves some extra plumbing that's only feasible if you are building a new house (like some of you are planning to do).