The typical height for a jump over solid ground to be more or less certain death is 80-100 feet iirc. If you're jumping from one place on the ship to another that's all you'd need. If you were jumping in the water, you'd need to be a bit higher for the same effectiveness (not sure how high exactly, you'd have to research).
I believe jumping into water with a weight tied to you would be the best option. Drowning on it's own is second only to firearms in lethality. So, coupled with falling- even if you didn't die upon hitting the water, there's a decent chance you'd be unconscious while you drowned (which you would almost certainly not survive), and since you'd be sinking there'd be even less chance of medical attention.