1. Do not travel with a hard suitcase, a good travel bag is better to store.
2. A passport, if you need one for the planned trip. Otherwise, an identity card is sufficient. (Inform yourself in advance)
3. A coffee-to-go mug to drink your tea or coffee (not a must) or a water bottle to refill.  
4. Sailing clothes are not necessary, but a good rainproof jacket, rain trousers and sturdy shoes are always good. Be it for walking, cycling or helping on deck.
5. Don't forget sunglasses, sun hat or cap, and sunscreen, even when the sun is not visible you can get quite burnt.
6. A good book for a bit of solitude or a good party game to play together with your fellow sailors.
7. Photo camera! So you can capture all your memories and share them with those back home.
8. Cash money, because here you can only pay your drinks bill in cash. But also remember the currency of the country you are travelling to, so you can buy a souvenir. 
9. A parlour game to play with fellow travellers
10. Good humour! (very important)