They have a high water content. They are the best source of protein. They are rich in minerals and vitamins. They are the best source of carbohydrates. On cold days, snakes usually lie very still and eat little or nothing, while birds usually move around and eat a lot of food.
More oxygen production occurs near the surface because there is more light there.
More oxygen production occurs near the bottom because there are more plants there. The greater the water pressure, the more oxygen production occurs. The rate of oxygen production is not related to depth. The melting of ice. The grinding of salt crystals to powder.