Dock side water not working

Ahh.

That black gook is most likely came from your dockside hose over the summer and over time. It builds up when the water sets in the line with no movement. Such as when you leave it for the week or whatever.
The gook/skank will come loose from your dockside hose whenever you walk on it or move it around. It will then collect at the check valves and even stop up the inlet valve.
I have an 1981 Ocean with probably the same plumbing lines you have, the plastic grey stuff. I have never seen and gook/skank in my boat plumbing lines while working on them .
Your line and water tank most likely are fine. Don't mess them. I have had my dockside water hose clog up my inlet and check valves.
Two years ago I installed an In-line water filter in the engine room just past the dockside water inlet and just after the water pump discharge from the start of the tank so I am filtering both dockside water or tank water.
I got it from Home Depot. It's a removable filter type see through, not a plastic element cartridge. It work real well and filters out the gook/skank real well.

I also drain my dockside hose when I leave for a number of days to keep the skanky/goo/poo from forming.

Clorox is only a 6% solution to begin with, coming from the shelf. Pool chlorine from the supply house is perhaps 12-13%. Chlorine the municipalities use for their treatment facilities is about 15%. Adding a small amount of it to the tank, running the boat for movement, flushing the lines, letting it all sit, and then purging and flushing is not a bad exercise for a system in poor shape. You certainly would not need to add one gallon of Clorox for every 10 gallons of water. A cup of it per a hundred gallons is likely more than enough to do the trick.

What % is OK to drink? What % will damage polybutylene tubing? What % will clean your water system? What % will kill algae or mold and what % will rid the system of the remains?

In spite of running the water from the tanks regularly and filtering it when filling, we also have periodically used a light bleach mixture in all of our boats. We never had an issue with water quality that way.

And when we were on Long Island Sound all year long, we used vodka when winterizing the smaller boats. Makes the spring commissioning much easier!

Did you put the Vodka in the boat, or ....

I usually use a quart for 250 gals? as an overnight treatment? My earlier comment didn't mean to imply 10% by volume I should have been more clear.

Clorox has a lot of information about how to use the product for different purposes on their website. Also, the best product for a purpose. Clearly, the perfumed variants of their "Bleach" products would not be ideal for potable water. Many of their "bleach" variants have additional chemicals for different purposes. As rtrafford says, Clorox plain bleach is around 6% Sodium Hypochlorite, varying from 5%-10%. When you buy that off-brand super-cheap bleach, it may be less than the name brand. Pool chlorine will be more.
The MSDS for Clorox is HERE
As cleanslate notes, dockside water hoses can promote algae growth. I discovered that big chunks of green algae would come out of the white "yacht" hose when washing the boat, and the white hose itself would develop a green caste to it. Opaque hoses didn't do that. That was when I ran a yacht in Ocean Reef, but never saw it in San Diego.
At one time, I filtered the incoming water to the tank with a 5 micron particle filter, and a carbon filter. Turns out the carbon filter was not a good idea, as it removed the chemical treatment that prevented algal growth and bad taste and odor. The 5 micron is a good idea as you will see how much algae comes from the dock water or hose, and will keep sediment from entering the system and settling out in the water tank and the water heater.
One season in Alaska, we used nothing but Reverse Osmosis water, and by the end of the season, I could see algae build up in the clear strainer from the tank. I switched back to dock water, and the municipal treatment was enough to clear up the bloom.

Why are you guys waisting good or bad Vodka ?!?
Pink stuff is fine and more inexpensive at least for winterizing...

Save the darn Vodka!

All this being said.
I recently put in a new tank gauge in my water tank.
So doing this when I remove the gauge from the top of the tank I could easily inspect my water tank which I have great access to from the top.
my boat is a 1981.
here is what I found in the tank absolutely nothing other than some Sandy sediment at the very bottom which I sucked out with a shop vac.
my tank is fiberglass and it was clean.
Perhaps lucky me but I would say this is the case for most boats.
Empty the tank and run a few tank loads through it to get rid of the smell and odors in my opinion.

Tell me more about the Opaque hose...
Do they sell them or do I need to buy the hose and make it up myself?

Its an old trick, per pink stuff ....

I've been wearing my "opaque" hose since my wife found them rolled up in the back seat of my VW ....

And I've heard they look very sexy.
Back to the thread however. Although algae loves the light mold loves the dark I believe.

I use a white, potable water grade dock hose here in the NE. I don't ever recall black gunk coming out of it? I leave it attached to the dock spigot and coiled on a holder. I use it at least once a week. Same with my cockpit hose. But after following this thread, I'm going to go down in the ER and examine my dock water valves and connections - see what I find. Maybe I have a problem and didn't even know about it.

The white (with stripe) is a potable water hose, meaning that it doesn't leach chemicals from the hose into the water. It can still get mold and algae. I'd recommend wiping it down and flushing it with a bleach solution at the start and end of each season and storing it thoroughly dried in a dry location.

Gotcha, I just have never seen any evidence of that growth?

It might be too cold where you’re at. Here in South Florida I have had issues with algae growing in several brands of the white with teal or blue stripe or etc. I think its hotter here and the water in the hose gets so hot it kills all of the chlorine in it (from the city).