Although it may look like sewage, and even smell like sewage it may be rotten bilge water. Many boats have a very small "catch" basin for showers. There is a bilge pump in these plastic boxes which have a capacity of a gallon or less. When the shower is being used the basin fills and trips a bilge pump which pumps the grey water overboard, (it is legal). These catch basins often overflow, they are designed to be not watertight. The assumption is that if the pump can not handle the volume while it is at its peak the rest ends up in the bilge and then overboard.
This foul "sewage" you have discovered may just be stagnant bilge water. It can get to be pretty smelly. Remember it has hair, body sweat, other body fluids, oils, etc. in it. And if it is stuck in an enclosed space, it's full of nasty bacteria.
It should be handled the same way you would handle fluid from the black water tank. It can not go into the marina water!
But my point it this. If you accept the job, do not limit your search to just the holding tank and the toilet hoses. It might be the shower plumbing. In fact, if the boat is in salt water it very well might be a hull leak, salt water is full of little animals which really stink when they die. It also could be a leak from above, like the deck. Any water allowed to sit in an anaerobic environment will smell.
This may even be a "Hazmat" type of job.
pete
