The svelte songstress participated in UNICEF's Celebrity Tap campaign, which asks stars to donate bottles of their own tap water to raise money and awareness for impoverished children in need of clean drinking water globally.
Rihanna stars in a stylish black-and-white PSA on celebritytap.org, in which she slinks in a clinging black dress down a hallway and into an ultra-modern kitchen where she fills a clear glass with clean tap water and sexily drinks it.
A breathless voice-over says, "Water so hot, it can only come from Rihanna's faucet."
The pop star's self-involved demands caused "extensive delays", sources said, holding up the project's launch for more than a week at a cost of more than $14,000.
"We're surprised to hear that," a UNICEF representative said.
"She was not unhappy. We were very thankful. Rihanna was very collaborative and very participatory. What was shot was what exists now."
The representative added that tweaks are "part of the normal creative process" and "no reshoots were done".
She said promotional work was done on a pro bono basis and any extra costs did not come out of the charity's coffers.
The Celebrity Tap campaign also includes celebrities Taylor Swift, Robin Williams, Adrian Grenier, Dwight Howard and Selena Gomez.
Since 2007, it has raised nearly $2.3 million for water and sanitation programs for children in Belize, the Central African Republic, the Ivory Coast, Guatemala, Haiti, Iraq, Togo and Vietnam, according to the organisation.
A representative for Rihanna declined to comment.