The perfect item of baggage for a sailing trip is a bag or a backpack. Be aware that you have to fold up your item of baggage!
Suitcases cannot be stored away aboard.
Basics:
Identity card / passport
Flight ticket
Cash money
EC card
Credit card
Driver license
Boating certificate
Short Range Certificate or Long Range Certificate
Wristwatch Waterproof
Mobile phone + charging unit
Practical items aboard ship:
Shoes for the yacht (they must not leave traces)
Flip flops
Sunglasses with dark glasses
Swim goggles
Diving goggles
Snorkel / flippers
Swimwear
Cozy wear
Bedroll
Kitchen towel
Face towel
Windbreaker jacket
Jacket consisting of fleece (advantage: fleece dries quickly)
Paracetamol / Aspirin
Possibly pills you need individually or special pills for journeys
Sun blocker with a high sun protection factor
Tweezers
Flash lamp and additional batteries
Clothes for the head (e.g. baseball cap or sun hat)
Personal necessities:
Diary for wonderful memories
Note pad for ideas and inspirations
Camera and additional batteries
Book for chilly hours
Favorite music
A photograph of your love ;-)