I've noticed that the bowl always seems to fill 1/3 - 1/2 full of water on its own, despite my pumping the bowl dry each time it is used. Over time, minerals have precipitated out onto the bowl and it just looks, well, unattractive. There is no smell to the water. I keep the water intake valve to the toilet closed, and the bowl still seems to collect water. At 1/3 - 1/2 full, it's okay... but if it gets higher than that, I don't want any spilling out if the boat is heeled over! Is there a valve somewhere that might have a worn/cracked seal that is letting water into the bowl? If it was back-up from the holding tank, I would expect it to smell bad. Any ideas?