Na verdade, Aguamenti fez de fato funcionar.
As proteções de Voldemort simplesmente desapareceram de qualquer água produzida (com sucesso) por Harry.
"Aguamenti!" he shouted, jabbing the goblet with his wand. The goblet filled with clear water; Harry dropped to his knees beside Dumbledore, raised his head, and brought the glass to his lips — but it was empty. Dumbledore groaned and began to pant. "But I had some — wait — Aguamenti!" said Harry again, pointing his wand at the goblet. Once more, for a second, clear wa-ter gleamed within it, but as he approached Dumbledores mouth, the water vanished again. "Sir, I'm trying, I'm trying!" said Harry desperately, but he did not think that Dumbledore could hear him; he had rolled onto his side and was drawing great, rattling breaths that sounded agoniz-ing. "Aguamenti —Aguamenti —AGUAMENTI!" The goblet filled and emptied once more. (Harry Potter and the Half-Blood Prince, Chapter 26: The Cave)
Embora não esteja claro a partir do cânon, eu não acho que qualquer feitiço "mais strong" teria funcionado - os feitiços para produzir água não são projetados ou preocupados com o que acontece com a água APÓS ela aparece (por exemplo, eles não criam água "não lavável").