Q.Ficus trees
I have a neighbor that cut down very large ficus trees and has left branches laying in the creek. Not just a couple but long footage across from 4 duplexes. Canal has inlet from river but no other end outlet.I am located in west coast of Fl. What type of damage are these dead branches and rotting leaves doing to the water quality and marine fish and creatures. Boaters are being careful when passing area but some may not be aware of anything has sunk to bottom.

I would contact your local County Extension Office and voice your concerns.
There may be local laws on how to dispose of yard waste.
Here is a link that will help you locate your nearest office.