I read once in a book by Sebastian Junger that the sea has four colours. There is white water, which is the crest of a wave; green water, which is the body; blue water, which is below the waves; and finally black water, which is the deep. If you’re in the white or the green, you can feel relatively confident about returning to the surface. Blue water is neutral: you’re properly submerged. Finding yourself in black water means you’re sinking fast.