Your vessel will have all important documents onboard. This will include its register list for vessels, insurance policy. You should check these documents before you set off.
It is best not to store your personal documents and the vessel's documents in the same place.
The clothing you should bring depends on the time of year. If you're expecting bad weather, or sailing in the low season, then waterproof sailing clothes and shoes are essential. If, on the other hand, you are sailing from June to August, a waterproof rain coat, a windbreaker and summer clothes will usually be enough – t-shirts, trousers, shorts, bathing suit, a hat, and a pullover for cooler evenings.
Appropriate boat shoes are also essential – trainers that are bright and have non-slip rubber soles are ideal. You will probably need a second pair of shoes for when you go ashore.
Your luggage is important as well. Hard cases are not suitable for sailing boats, due to the amount of storage space they take up. Instead, you should bring soft bags that can be easily stacked.