It has has been noted it is very very difficult to just fall off a modern cruise ship. It's even more rare to have a person thrown overboard by another person(s). In most documented cases it is found either that the person who went overboard jumped over themselves, was engaging in some kind of behavior that caused them to go overboard, or there were no witnesses, no evidence and no indication as to how the person actually went overboard.
the only case I'm aware of where a person at least attempted to throw somone overboard is the HAL case, in which the crew member decided the passenger had insulted his family in an offhand remark, stalked her around the ship for hours intending to assault her in retaliation before he figured there were too many people around, then hid in her stateroom (he was her room steward) and attacked her after she went to bed. He beat her up, raped her and tried to throw her off her balcony. but she resisted and he was not successful in doing so; in fact she managed to escape from him.
He was convicted on US federal charges of attempted murder and aggravated sexual abuse and sentenced to prison for 30 years and 5 months.
There was also the George Smith case where it's often alleged that the group of men he was hanging around with may have thrown him overboard in an attempt to rob his stateroom but nobody was ever charged and no one has confessed or anything like that.