I was able to find a few organizations that have been involved or plan on getting involved with water projects in Maasai regions.

1. Practical Action - I believe this organization focuses on sustainabiliy in all areas, and that they do things all over the world. One of the regions they work in though is Eastern Africa. They do not specifically have a water program, but water is incorporated into other areas of focus (ie food, soil, etc.) and since they have been working in the region for about 20 years, they might be helpful.

http://practicalaction.org/
More specifically:
http://practicalaction.org/?id=food_production
http://practicalaction.org/?id=water_and_sanitation_strategy

2. African Medical and Research Foundation - They are involved in the Magadi water, Sanitation and Hygiene Promotion Project and they project is aimed at teaching the community to sustain a healthy water source. The projects seems pretty new so I'm not sure how much information they have. Regardless, they may be helpful.

http://www.amref.org/
More specifically:
http://www.amref.org/index.asp?PageID=63&PiaID=6&CountryID=1&ProjectID=100

3. Earthcare Africa - They were involved in building a rain water harvesting system that will help the Maasai women obtain water during droughts. The project is announced here: http://www.unep.org/documents.multilingual/default.asp?documentid=264&articleid=3137&l=en

I'm not sure what the website for the organization is but the contacts might be in the article.

-Stella