To quote paraphrase mangomadness, this is a 'noooooo' thread.
I need to remove an illegal head on my boat. Direct discharge. Yes, I know some of you read the thread and debated it's legality. The gist of it is that IT IS ILLEGAL, I researched it and it's true.
I plan on fixing the head without taking the boat out of the water.
The outlet valve is a gate valve that closes fine.
The inlet valve is another gate valve that will not close. It has a length of garden hose attaching it to the head that is pretty long. I am going to dive under the boat in the slip, plug the inlet thru hull with a cedar plug, remove the upper hose clamp on the inlet hose, and drain the water. At this point, I am going to verify that the plug does not leak. So far this procedure is risk free as the hose end will be above the waterline.
Then I am going to remove the old gate valve, and put a cap over the thru hull.
I will have a few contingency plans:
Something to plug the hose with.
Plenty of different plugs.
If it works out well, I am going to do it with the outlet thru hull.