The shape of the water is decisive on how the light passes through it. Coming from an optically less dense medium (air) and entering a denser one (water), the light is partly reflected back while partly entering the water. Depending on the shape of the water, the light forms crinkle patterns or becomes diffused randomly in all directions.
The amount of light that is reflected upward depends strongly on the height of the sun (place on Earth, time of day and season) and the condition of the sea. A rough sea absorbs more light whereas a mirror-like sea reflects more. In the tropics, the sun stands straight overhead at mid-day, resulting in little loss. In temperate seas during winter, the light diminishes by as much as 3 f-stops immediately under the surface.
As a matter of interest, the reflected light is partly polarised (horizontally) and so is the part that enters the water (vertically). Polarisation is maximal in the early morning and late afternoon when the sun stands low in the sky. The vertically polarised light entering the water makes objects less shiny, more colourful, and can be used creatively, for instance to capture the deep colours of shiny fishes in natural light.