It is advised to travel with a travel bag instead of a hard suitcase, since it is better to stow.
A passport, if you need it for the planned trip. Otherwise an identity card is sufficient. (Inform yourself in advance)
Oilskins are not necessary, but a good rainproof jacket, rain trousers and sturdy shoes are always good. Be it for hiking, cycling or helping on deck.
Don't forget sunglasses, a sun hat or cap and sun cream, even when the sun is not visible you can get quite burnt.
A good book for a bit of solitude or a good parlour game to play together with your fellow sailors.
A camera! So that you can capture your memories and share them with those at home.
Cash, because here you can only pay your drinks bill in cash. But also think about the currency of the country you are travelling to so you can buy a souvenir.