Oceanography/Salt Water/Fresh Water
From what I understand, if you mix salt water and fresh water, the water would become hazardous to marine life. Since rain water is fresh water, would it not that when a hurricane comes through the coast and dump all that fresh water into the coastline that it would also kill the marine life in the area?
It's not that simple. In the open sea, the volume of the oceans is huge and during a large storm the surface layers are generally well mixed by wind and waves. The net result is a salinity that generally does not fluctuate enough to concern ocean life.
Now near the coast, the discharge of large amounts of fresh water from land - be it from hurricane rains or river mouths - can lower the salinity enough that many open ocean species cannot tolerate the conditions as you suggest. These are what we call estuarine conditions - the salinity is intermediate between "fresh" and "open ocean".
There is an entire suite of aquatic species adapted to such conditions. Many forms of marine life can also tolerate such conditions for a short period.
So, to get back to your question, hurricane rains might kill some marine life in a coastal area but which species - and how broad a an area is so affected - will vary with local geography, wind speed and direction, rainfall patterns, etc. so it is most difficult to predict or measure such impacts.