How about a few people, going on tropical vacations, take a testing kit.
Inland lakes occasionally become over populated with algae, due to many different reason, but frequently because of pollutants, "over feeding" the algae.
Frequently, the visibility of ocean water has to do with weather conditions and how much bottom sand/silt is being stirred up by the ocean tides, currents, waves, etc. So turbidity of water is not necessarily due to the presence of algae.
In healthy bodies of water there is a balance of organisms that keep the algae in check. Without algae, though, the oceans would probably die, beginning at the bottom of the food chain, where the smaller animals consume only algae. Larger consume those and then on up.
gg=alice