Why have a Waterlift Muffler?

One reason to have a waterlift muffler, according to John Mackie, John William #68, is to prevent water, which is aft of the muffler but has yet to be discharged overboard, from backing up into the engine. This could occur in large seas when the bow is down. Interestingly, Knock Off #66, has no muffler at all.