The Northern river runs into the Chetumal Bay at Corozal so naturally the water is murky ,but natural as in mud and silt from the rain forest river running into the Bay. I know of no industry up there that is causing pollution. There is a little agriculture run off from sugar cane fields and the rum processing plant that makes the yummy Belizean rum is on that river but I don't think Its anything like 'pollution' as one might perceive pollution in the states, and of course none of that is anywhere near Ambergris where the water is often so clear I can read the name of the boat waiting on me at the surface from 100' down. :-)