In the Night Sea

My father's hometown is a small island, and everyone would come back for grave visits in the summer, so I used to stay over and play with my cousins. It's an island with nothing but a single small store, so the only things to do were swim in the sea or catch bugs. But I always stayed overnight during my elementary school days. (My parents all went home.) One night, it was really hot (no air conditioning, just a fan, and the house still had a pit toilet), so my cousin and I snuck out to the sea at night. We weren't swimming, but just standing on the sandy beach was cool. As I gazed at the dark sea, I saw a large, black island. "I wonder which island that is? Maybe it's Island XX?" After chatting about things like that and cooling off, we went home and slept. The next day, when we went swimming in the sea, there was no island there. It was just the ocean stretching out. Was it something that came back for Obon? Or was it the legendary sea monk (Umibozu)? The truth is shrouded in mystery.