"La mar" is the feminine name for the sea, "what people call her in Spanish when they love her." Santiago felt about the sea as he would feel about a beautiful woman, one who changes and challenges and may cause great grief, but may also give great pleasure and yield wonderful treasures.
Hemingway portrays Santiago's deep awareness and appreciation of the beauty of the sea and the life in it. The colors and contrasts and changes as Santiago studies the water convey the great joy he derives from noting all the nuances of appearance and movement.
The water was a dark blue now, so dark that it was almost purple. As he looked down into it he saw the red sifting of the plankton in the dark water and the strange light the sun made now. He watched his lines to see them go straight down out of sight into the water and he was happy to see so much plankton because it meant fish.
Santiago's affection for the sea is reflected in his intimate knowledge of the water, the plants and animals living in, on, and above it, the weather conditions above it. His pride in being a fisherman is based on his alliance with the sea, the way in which it supports him and in which he respects all that comes from it.