The problem is not a financial one. Money is not the constraint. There are people with access to (for our purposes) unlimited sums, with good intentions, and a good track record at getting difficult things done, who have not made a meaningful dent in the problem.
The answer must lie in an approach nobody tries, which immediately eliminates throwing money at problems. It also means that it doesn't make sense to give money to an existing organization.
The problem is inherently informational. How many thousands of small villages have wells, but no access to clean water because some small, inexpensive part is broken? I've travelled extensively in the third world, and this is the situation in all too many places. The key is in knowing what is needed where, and what equipment and knowledge is required to redress the problem.
The right answer is some sort of mega-clearinghouse of such information; matching people who know what is needed where with people who are headed in that direction and can bring equipment and acquire the knowledge required to successfully deploy the equipment before they go.