The clarity of sea water is very dependant on the concentration of TINY TINY particles of stuff floating in the water. This stuff can be suspended grains of minerals that get particles of biologic origin.

When sunlight penetrates the oceans, the light is scattered by these particles. That is, the light rays bounce off of the particles ... or reflected off of the particles. The reflected light is scattered in all directions and this makes the water lose its transparency. After big rain storms in Santa Barbara, California, you will note that the water looks dirty brown within a few hundred meters or more out to sea; this is because huge numbers of tiny clay particles are suspended in the WATER COLUMN and that makes the water look murky.

You can do an experiment at home.
Collect a few grams of soil somewhere and take a bottle of water, put the sediment into the water and shake vigorously. Then shine a light through the bottle and on the opposite side monitor the amount of light that is getting THROUGH the water. You will note that after a few hours more and more of the light can travel through the water.