Lord of The Rings: Fellowship of The Ring - Bone Dry
In the scene where Frodo is setting off alone on his journey to Mordor, Samwise chases after him into the water. He nearly drowns before being pulled into the boat by Frodo, and although his hair is wet, his cape is bone dry!
This is not a slip-up. Anyone who read the books would know that there is a lot of magic in those elven cloaks. Very little is shown or talked about in the movie other than the scene where Sam and Frodo hide under the cloak and it looks like a rock. That is just a sample of the magic. In the books, Tolkien mentions that the exact color is difficult to describe because they seem to change color depending on the surrounding environment...everything from a pale grey to a dark brown. It is not hard to imagine that the cloaks are magically water resistant since they are worn over all your other clothes to keep you warm and protect from weather.