In folktales from various cultures there are tales of sea animals so big, that they get mistaken for islands. In some stories, sailors land on them and spend time on “land” before realizing that they are actually on top of a living, breathing animal. These islands accordingly appear and disappear, rise and sink into the ocean frequently. In the Irish Legend of Saint Brendan, Brendan of Clonfert, a monch and fabled navigator, and his disciples land on a giant sea creature called Jasconius. “Because of its size, Brendan and his fellow voyagers mistake it for an island and land to make camp. They celebrate Easter on the sleeping giant’s back, but awaken it when they light their campfire. They race to their ship, and Brendan explains that the moving island is really Jasconius, who labors unsuccessfully to put his tail in its mouth.”
In mythology sea animals often trick humans into believing that they are on safe, solid ground. But equally often, they rescue people from floods and offer their bodies as rescue boats. See for example the tale about the fish Matsya here.