Questions of pollution aside, the answer, oceanographers say, is that cold northern waters tend to teem with great concentrations of tiny plants and animals (phytoplankton and zooplankton), while southern waters are (in terms of numbers of individual organisms) empty deserts, with a few species-rich oases, such as coral reefs and waters fed nutrients by river runoff.
Phytoplankton need sunlight, nutrients, and carbon dioxide to thrive. Cold waters are richer in nutrients like oxygen and in carbon dioxide because the water molecules move more slowly than they do in warmer waters, allowing more dissolved gases to stay dissolved.
The result is seasonal crops of the phytoplankton, which in turn feed the zooplankton.
This rich soup of microscopic plants and animals and decaying plants makes cold northern waters appear murky. By contrast, a tropical sea has almost nothing suspended in the water to refract light or block the view.