I heard that it is very common for people to die on cruise ships. So common that I understand all cruise ships have a crematorium. Since most of the people on cruises are usually old and some of them go on them for that reason.
So is a kohen allowed to be on a cruise ship?
So, a person could become tamei meis when being Ohel, being in the same room as a corpse. If in the same room, a corpse can transmit tumas-ohel.
Rav Moshe Feinstein ruled that a Kohen should not stay in a hospital where he is forced to stay even if the body of a Jewish corpse is present.
He says that a kohen may only accept such a job if he clearly stipulates that he will be allowed to immediately run out of the hospital if he becomes aware of the presence of a Jewish body.
See footnote 8 where this ruling is sources to Igros Moshe Yoreh Deah, Chelek Aleph, 248.
Is running away possible when being on a cruise?
With regard to a cruise-ship, it is known that a cruise-ship have morgues in it. Usually they are located at the lowest ship deck, far away from the people on the ship.
Tumas Ohel is contracted by being in the same room as the corpse. So as a Kohen on a cruise-ship, one should avoid such places. I do not see why a person, let alone a Kohen, should even visit such a place in a ship. They are simply not allowed.
The simple way that tumas ohel is contracted is by being in the same room as a corpse. Most Poskim are of the opinion that if there is an open window between the room that the body is in and the room that a kohen is in, he will become tamei d’oraysa as well 1. There is another way that a kohen may become tamei through tumas ohel, which is known as “sof tumah latzeis”, the way in which the corpse will be taken out of the building. This means that if the corpse will be taken out of the hospital through a specific hallway or doorway, that hallway or doorway now has the status of tumas ohel even before the body is taken out.
According to this, a Kohen should not be a) withing the same "roof" of the place where corpses are, b) in the near area of sof tumah latzeis, the way in which the corpse will be taken out of the ship.
The Shulchan Aruch (Yoreh De'ah 371:1) rules:
A Kohen is forbidden to enter beneath an overshadowed space under which there is a corpse, even if it is very large; and [he is] even [forbidden to enter] into another [adjoining] house or another [adjoining] upper storey, which open into that house by means of an opening a square handbreadth in size, and [the prohibition also applies to entering] a house [which opens] into that [latter] house ad infinitum.
[...] footnote 3 If a corpse lies in a room, whatever is within the room suffers corpse-uncleanness, and if the room has many doors or exits, even if they are closed, all objects which lie beneath the lintels of these doors or exits become unclean, because we assume that the corpse may be removed through any one of the exits, at which time the corpse and the objects will be together under the lintels, and the objects will suffer uncleanness through overshadowing (v. supra n. 1a). Hence, we regard the objects and the corpse as if they were already present under the lintels of the door or exits. But if we know through which exit the corpse will be removed, then only that particular exit suffers uncleanness, not the others. This means that that particular exit saves the objects beneath the other exits from uncleanness through overshadowing. The minimum size of such an exit must be one handbreadth square for an olive’s bulk from a corpse in the room, to prevent uncleanness from spreading to the other exits, and a minimum of four handbreadths square opening will prevent the uncleanness from spreading to the other exits in the case where an entire corpse is present in the room.
So, will tumas meis spread to other rooms? Well, according to the above, that depends on whether there are any windows or doors that connect them to eachother.
With regard to a shul that was located near an appartment, Rabbi Yirmiyohu Kaganoff writes:
A shul was located in an adjacent building that connected to the apartment where the meis was located. Could kohanim enter the shul to daven on Shabbos? Since I could not ascertain whether or not there were any openings between the two adjacent buildings, I advised that the kohanim not enter the shul until the meis would be removed after Shabbos.
This could also apply to a cruise-ship, right?
Needless to say, if it is a really small boat, the Kohen is not allowed to step even into it. If it is a big ship, e.g. a cruise-ship, the Kohen isn't allowed to be less than 4 cubits away from the area. (see Siman 371:6)
