Fresh Water Meets Sea Water
This is the Fraser River Delta just in front of Vancouver BC Canada where sea water meets fresh water. The river water from Fraser River, flows into the ocean water of the Strait of Georgia. Modern Science has discovered that in the places where two different bodies meet, there is a barrier between them. This barrier creates a noticeable line, as each water body has its own temperature, salinity, and density.
it used to be pretty common for there to be cats on boats. they'd take care of any rodents who would chew on ropes or wires and spread diseases. sailors were also superstitious and believed that having a cat aboard would bring good luck! this belief passed on to their wives, who kept cats - especially black ones who were believed to be extra lucky - at home in order to keep their husbands safe when at sea.
another popular superstition? that if a cat came aboard it was a sign of luck, but if it only boarded halfway and then left, it was a sign of bad luck.
most ship's cats are only found in modern times on private vessels, but they have roots going back to early history. one such example is the Vikings, who took cats with them on expeditions.