Hi,
Iohannes considers the ship his sanctum, though he can easily lower the banner (the sail is lowered in port) and cover the marker on the hull, if a visiting magus feels nervous. Though Iohannes will point out that a magus who abuses his sanctum privileges by killing a magus guest is not likely to live long and prosper once the Quasitors explain to the Tribunal what happened and various more powerful magi decide to express their offended sensibilities in a tangible fashion, the official outcome of the trial notwithstanding.... unless, of course, the slain magus has enemies and no friends or patron.
Considering the potential of a Sight PeHe or CrIg cast at sea, a huge and prominent sanctum marker has its place.
grin Thanks again for your questions!
Anyway,
Ken