User blog comment:Angren Túrë/When's the Darkness gonna show up?/@comment-4010415-20151102194138/@comment-26397825-20151104015604

Basically what 72.Anon said. The Inferi stayed underwater, let anyone cross it to the island, but as soon as someone touched the water to get a drink for the poor person who drank the potion in the chalice, the Inferi came out and tried to drag them under. They couldn't summon water because the island had a spell on it that vanished all summoned water.

And yes, it happened in the books. I haven't seen any further than the third movie, but I've read all the books.