Sea fairy cookie - Seamoon

The moon's gravitational pull affects the Earth, drawing the waves on the ocean towards it; thus, when the moon is closest to one side of the planet, it will be low tide there, and high tide on the other, and vice versa when it travels to the other side.

Seamoon continues to gain popularity as time goes on--particularly with the series' newfound popularity following Kingdom--as more people come to learn about Sea Fairy's love for Moonlight.

Despite them never mentioning Moonlight directly, many of them reference her missing someone or recalling something in her past: presumably Moonlight, since she is an integral part of Sea Fairy's story.

It is very likely that Sea Fairy and Moonlight's story is influenced by this natural phenomenon, as Sea Fairy tries to reach for the moon--effectively raising the sea and causing low tides where she is--when it's in her sights.