Definitely agree with Claude A. The head inlet is probably close by ..under the washbasin? in the bottom of the hanging locker ? in the engine bay ?? under the bunk aft?
Sounds like you have a piece of debris stuck in the inlet line check valve of the head pump. The valve should be closed by the "dry" position on the head. You can try going to "wet" and giving it a couple of hard quick pumps.. ( I am assuming a regular manually pumped head and not an electric one) then back to "dry" and clearing the bowl .. If that doesn't work, you can cut off the flow by squeezing the inlet hose to the pump.. (Not the hose going to the bowl) Wrap the hose in a wash cloth and use vise grip type pliers to squeeze the hose. This will give you time to fix the problem before the boat sinks ! You may have to replace the hose after shutting it this way.. An alternate would be to get a tapered plug about hose size and pull the hose off the head.. At that point, you will have full water flow from outside the hull, so be prepared to get wet and be ready to put the plug into the full flowing hose.. quickly ! DO NOT leave the boat until the plug is secure 'cause if it comes out, the boat will sink quickly.. If you aren't comfortable doing this, get a boatyard guy to shut the leak off .. this kind of leak will sink your boat !
