Marketing is the art of telling stories so enthralling that people lose track of their wallets. And every marketer knows such stories can’t be too complicated. So another tenet at Charity: Water is that solutions need to be presented in simple ways. “Our approach is that water is binary,” Mr. Harrison told me. “People are either drinking clean water, which is good, or they aren’t, which is bad. We want to present an easy choice.”

That strategy — which the group propagates through online ads, social media campaigns, direct email solicitations and even the occasional billboard — has been remarkably effective. Over the past decade, Charity: Water has raised $252 million and has supported 23,000 projects in villages and rural areas across Africa, Asia and elsewhere.

It has become a cause celeb among tech entrepreneurs, Hollywood stars and the Twitterati. Unusually, 47 percent of its donors are millennials (most charities struggle to hit 10 percent).

But Charity: Water has also drawn criticism from people who feel the group has oversimplified a complicated problem. Water isn’t actually binary. Is it unequivocally good if Charity: Water builds wells in areas where terror groups are active? Is it wise to emphasize water over other philanthropic causes like vaccines or schools? Does it make sense to build wells in remote regions, where Charity: Water works, rather than large cities, where many more impoverished people reside?

“There’s lots of people who think Charity: Water is working on the wrong issue, or they’re telling the wrong stories,” said Patrice Martin, who helps run IDEO.org, a nonprofit organization that has worked with Charity: Water. “But what they’ve done is get Americans, especially millennials, interested in the lives of people on the other side of the world. We should be learning from them.”

That said, it can be hard to see how to apply some of the group’s lessons to really complicated issues, like Syrian refugees.

“With an earthquake, it’s easy; everyone is an innocent victim, and they have a problem that will eventually end,” said Amanda Seller of the International Rescue Committee, who notes that even the phrase “innocent victim” unfairly segments philanthropy. “Wars and man-made disasters are really hard,” Ms. Seller said, “because sometimes you can’t tell the victims from the perpetrators or how long it’ll last.”