The Victim (lets call him Z for ease, I suppose) boarded the ship with fellow employees A, B, C, D, E and F on the first day of the cruise.
At some point during the day, Z was either intentionally or accidentally responsible for a incident which critically injured one of the other six, potentially even to the point of a coma. The narrative and truth to this point would not conflict the idea that one of the six of A-F were unable to act during this entire story thus far.
It was after this incident that Z lost his job and was no longer part of the company. However, obviously, he can’t just up and leave the boat.
Someone among the remaining five believed that Z had not paid enough of a punishment, this person maybe being very attached to the injured employee. They decided to enact the events of Pictoshark’s given scenario.
Z of course knew immediately that he was stabbed, however he either realized on his own or was told by the culprit of the intent behind the attack, and believed he needed to pay his dues. This is why he neglected to call for first aid until the morning.
I present this scenario as an answer for Why did the victim not alert anyone of the wound till the morning?
At present I am not attempting to answer how the attack was carried out and therefore have no need to explain how the culprit entered the room.