On my pretty stock 496 HO from 2007 with GenIII fuel cool, it's hard to get to the sea water pump. While working on that, I noticed that halfway on the thick water inlet hose is a black plastic fitting. What is that, a filter? Couldn't dismantle right then as the boat is in the water and I ran out of plugs...

Do you have a freshwater flush connection where you can hook a garden hose up?

No, I don't