You could set an entire Hitman game on a boat this big. Imagine having like a dozen targets you could take on in any order - the game takes place over a multi-week cruise, you want to make the deaths look like accidents so you don't spook the other targets, the crew could progressively beef up security in response to your actions, targets will attempt to flee the ship and you can pass or fail in trying to stop them, eventually the whole boat gets locked down and you have to evade special forces guys trying to find you as you take out the last couple of targets that remain.

Big replayability as you try to get the absolute perfect score of successfully getting every target with an accident in one run, game ends with the CEO of the cruise company being charged with gross negligence and blamed for all of the rich ghouls you killed. It's 10/10 in my head.