Keep through-hull closed
If you leave the seacock after the macerator closed, as you should do if you don't want the holding tank to fill, then the risk should be limited to a stinking mess, not a sinking boat. I'm not sure exactly how it relates, but the first thing that came to my mind was the fact that we had about a 50% failure rate in the consumer plastic containers on board when we cruised the Bahamas (detergent, cleaning solutions, water jugs, etc.). Fortunately, no oil containers broke, although I know this did happen to other cruisers. My theory is that these plastics are designed and tested for the stable environment of the store shelf and the home shelf with an occasional ride in a truck or car. When subjected to constant motion for several months, the slight movement of the liquid inside causes fatigue cracks in the plastic by constantly bending it back and forth. Intuitively, I would be inclined to switch to the flex marine plumbing eventually. Just don't skimp on the quality. I installed what I thought was regular marine head hose a couple of years ago and had to do the job all over again last week because the hose had collapsed and completely flattened on two turns. Regardless of quality, avoid sharp turns (i.e. where you have to apply strong pressure to get the hose to turn). Note: A hair dryer to heat and soften the hose ends helps tremendously when trying to push them over barbs.
