It's not supposed to hold water
Although some high-end electric toilets are designed to bring water into the bowl and hold it, most marine toilets are not. Most manual marine toilets have piston/cylinder pumps that all work pretty much the same way: a piston rod inside a cylinder pulls water in one the up stroke and pushes it out on the other stroke. So if it ever did hold water ahead of use, something was preventing the bowl from emptying. However, water brought in with the up (or down) stroke SHOULD remain in the bowl until the next the next down (or up) stroke...if it's disappearing before you can ever pump again, it means the joker valve in the discharge is worn out and needs replacing. A joker valve is rubber cup-shaped doodad that has a slit in the bottom and "lips" on the outside of the slit. The flange on it also serves as gasket between the pump and the discharge fitting. It's a one-way valve that prevents backflow from FLOODING the bowl, but it's not supposed to prevent seepage. Over time waste going through it stretches the slit and "lips"...till the slit isn't a slit any more, it's a hole...as a valve, it's useless. Average lifespan is only a year or two..best course is annual replacement each spring.
