Answer:
Irony
Explanation:
"the expression of one's meaning by using language that normally signifies the opposite, typically for humorous or emphatic effect."
I believe the correct answer is to help the reader visualize a view where water and sky are difficult to tell apart.
The author is trying to portray an image where the water and the sky are somehow blending into each other - you can't tell where one ends and the other one begins, because both items are colored similarly - various shades of blue.
Maybe it's because we hear it from others everywhere we go. We probably use it to sound cool or to build more words in a conversation.
