hope it isn't a PELNI ferry.......................

So does provision and servicing of a toilet. Yet oddly airlines don't charge for that. Why not?

You can go several flights without water and waste and it is usually covered by ground handling as a fixed cost included in the contract so comparing toilets to baggage is rather moot. Depending on the routing and time of day theta one water and waste visit will serve 600+ pax. It will be done every night and then usually at a crew change or at the request of the crew. You can't compare the two.