You do not need your passports. You can & should leave them in the safe in your cabin. You probably need a photo id & you musy have your ship's ID sign & sail card to get on & off.
You want some $$ in smaller bills & 1 credit card, preferably not the same one you secured your on-board account with just in case.
I usually bring a bag/back pack with some water, a towel, & sun screen .