Ok, put bluntly large amounts of poo dumped very quickly creates large concentrations of very bad things (bacteria, viruses, etc) in any water
Small amounts spread over time (especially with tides, currents, etc) don't allow for a heavy concentration to build up.
As stated previously, in fresh water (or brackish water like the Ches. Bay) most of the bacteria will thrive for a while, but will (for the most part) be out competed by the local stuff. Anything that manages to make it's way into the bottom, esp mud.. will last longer.
water does nasty things (ie causes H2O to move out of cells to try and equalize concentration) to most bacteria. Those that live in salt
water have to compete for food
with everything else already there and usually don't last very long (~2-5 days) but still stick around in quantities that your body can usually and easily defeat without your noticing.
It comes down to the level of concentration of 'poo' in the water as measured in several ways.
If there is a sewage spill in your vicinity, please leave for your own health
and come back later!
This being said... I agree that boaters should
use something like an lectra-san or a holding tank
to treat the viruses and bacterial .... BUT!!!!
All those quaint cities who claim they can't afford to stop using combined storm/sanitary sewer systems (particularly in the NE USA) should stop whining about what boaters MAY be putting in the salt water and fix their own systems!
If a sewage treatment plant breaks down they HAVE to dump it! It's not like you can just tell everyone upstream to hold it and don't flush! However they should face fines for dumping related to the volume that they dump and it's concentration (there's a lot of difference between liquid poo and mostly dewatered poo!)
What is more of a problem is the fact that your poo (Everyone reading this) being that you are more or less the top of the food
chain, has an increased amount of heavy metals and pharmaceuticals that CAN'T be removed economically at this time. The biggest offenders in our fresh water supply are female hormones (from birth control) and antibiotics.
So, Look around out... couple of boats, incoming tide or good current
, no problem... Lot of boats... high tide... you might want to move on or wait for a bit if your really worried or get sick REALLY EASY. Everyone else, jump in the ocean and have fun...
I am a civil engineer
. I make sure you have clean water when you turn the tap on and that the poo vanishes when you flush the commode, among other things... some of us even manage to keep places from flooding on occasion and we are usually responsible for acres of concrete appearing all over the world!
PS (seriously, if you have questions ask....)