and I'm going back 30 years to when I worked there, they fitted two. The second was a backup. If the first one failed, it could be cut away and the backup seal slid along the shaft to take its place. A wet proposition, no doubt, but feasible.
I don't recall the brand of dripless seal, but they were relatively new on the market. The rubber diaphragm (I guess you'd call it) turned with the shaft and abutted a disk fixed to the stern tube.
I remember hearing reports of ice being a problem with some dripless seals. Chesapeake Bay, where many boats stay in the water, can freeze occasionally, and if the water inside the diaphragm freezes, it expands and forces the diaphragm off the seal face. Have not heard any stories lately, but this past winter would have been a good test . . .