Nikos and the Sea God, by Hardie Gramatky (New York: G. P. Putnams, 1963. Out of print, but available used.)
For early readers, or reading aloud. Nikos is a young boy in modern Greece who believes he sees Poseidon in the sea storms; when a crisis arises, he meets the God face to face on the open sea. I was particularly struck by the fact that all the villagers were quite ready to accept the possibility that Nikos had in fact seen Poseidon, and when he accidentally fishes up an ancient vase with Poseidon’s image on it and a storm follows, their reaction is to assume that the God is upset and wants His property back. All in all, a nice story for younger kids that presents one of our Gods as real, without comment of any kind.