Question: Before gravity was understood, what did the ancients think caused tides?
Answer: Some people explained the motion of the planets by assuming that the planets were being pushed by angels. The same may have been true for tides. In other words, the explanation was found in supernatural forces. Since the Mediterranean Sea is very little affected by the tides, the ancient Greeks and Romans paid very little attention to it. Plutarch and Aristotle recognized the Moon as the cause of the tides, without understand gravitational pull. Pliny believed that the Moon had an attraction which caused the tides. Other scientists, like Ptolemy, settled just for a description without searching for a cause.