UPDATE:
I'm finally able to go out on my boat again and I was able to make some progress on my wash down pump setup to make it into a dual purpose fresh/salt water wash down. Pressure is about the same coming from the bladder connected directly to the pump as it was from the thru-hull, so I can eliminate that as one of the causes.
The main components used were a 14 gallon Lalizas Rectangular Flexible Water Tank, Part number 31321, and a Y-Valve from The Bosworth Company # 94AB-3/4-CPH. These come with different size hose barbs, I chose the 3/4" since that was the existing size hose for my wash down pump. I did have to change one of the barbs to 5/8" since that is the size on the flexible tank.
Tank:
Valve:
Here is a view looking down the hatch:
Here is a view of how the Y-Valve is connected, its within easy reach to make the switch from fresh to salt:
Finally, a look at how it fits behind the gas tank:
Currently I am filling the bag using the vent plug, I only add about 5 gallons so its been ok. Eventually I want to connect a fill connection to the leaning post so I can connect a hose and fill the tank without having to go through the hatch.