As an ex cruise ship employee, let me give you some stuff to work with!
Water tight doors! You get a special training video on interacting with these correctly because they will literally cut you in half if you try and go through them while they’re closing!
Freezer vaults for food in the sub decks - you can only get into these with the correct code and they have very thick walls. Good luck if you get shut in one of these just after the last round of checks bucko
There are cameras everywhere…except in the crew cabin corridors. Also there are no windows down there because unless you’re an officer, you live below the waterline. Day and night have no meaning because everything is in the same slightly unsettling yellow light.
Don’t piss off the guys who deal with the rubbish. They have machines down there that can crush metal barrels
As well as morgues, cruise ships usually have one basic operating theatre with all the attendant horrifying equipment in it
One cigarette thrown carelessly in the wrong place WILL start a fire that will gut half the ship.
When we’re pitching side to side, the anchor swings out and then back in, striking the metal outer shell with a noise that shakes half the ship
People disappear overboard more often than you’d really want to be a thing
A lot of cruise ships now have theatres on board (usually towards the front) with all the potential for dark corners, creepy costumes and electrical calamities you could want.
And as op says, you can’t really escape a ship in the middle of the ocean. Particularly during a storm, as then you can’t even evacuate to lifeboats unless the whole ship is going down. On the upside being on board during a storm means most guests hide in their cabins and the staff walk around like drunks, which would likely throw off a skilled murderer’s plans.
