The importance of water transparancy
Water transparency is a key factor in ocean ecology as the sun is source of energy for all biological phenomena.
Transparency reduction is due to the presence of particles in the water. When light attenuates, it alters or limits the capacity of life of some of the biological communities that live in the sea. It reduces the possibility of photosynthesis (a process that produces the oxygen vital to the ecosystem). It implies as well a reduction of visibility which prevents fishes and zooplankton to see their prey and their predators. It has also some considerable effects on human perception of recreational water bodies, fishing or even health. When there is a harmful alga bloom for example, mussels cannot be collected because it would be dangerous for the health. This has important economic consequences.
Water transparency is therefore often used as an indicator of water quality.
Measuring water transparency
There are different ways to measure water transparency. The eldest one is the Secchi disk, dating back to the XIX century, which is a white (and black) disk of 20cm diameter hold by a line that is lowered into the water until it disappears from the sight. The maximum visible depth indicates the transparency level of the water called the Secchi Depth. A demonstration of its use can be found in the video below (starting at 3:00 mins).
There also exist some oceanographic equipments (radiometers) but as they are very expensive, they are not adequate for a citizen science project.
More on water clarity
An interesting resource for information about water clarity can be found at: http://www.clarityonthesea.org/. For a quick impression please view the video below in which experts Marcel Wernand and Oliver Zielinski provide a good insight in the concepts of water clarity.