A friend of mine has just completed a purchase on the Casares del Sol urbanisation in the Casares coast. We are now busy connecting the electricity and water. The designated water company for the town of Casares is Acosol. The interesting and bizarre thing to note is that the main office of Acosol is in Elviria, past Marbella and on the way to Calahonda! The main switch board number is telf. 952831274. One can visit their office on weekdays Monday to Friday from 9am – 2pm. 
Alternatively Acosol have a satellite branch in the Marina de Casares urbanisation on the Casares coast. This office is only open on Monday’s and Wednesday’s from 9:30 -1:30 Contact telf. no. for this office is 952839329- probably best to phone them before hand.
Once you make it to either office. Please remember to take:-
1. Rental contract or title deeds
2. Licence of first occupation / Licencia de primera ocupación
3. Copy of your Passport/ ID card and NIE number
4. Bank details to set up the direct debit
5. If the house is brand new, you will need to take the original certificate of installation. ( Known in Spanish as the Boletin)- if the house has already had a water contract, you should take a copy of the old bill.
Please let me know how you get on.