You need to have the basic stuff with you like going to any sea holiday when going to a blue voyage. Sun cream, mayo, towel, sun glasses, hat, snorkel, rift swim mask are the must haves for a sea holiday. You will not need more stuff than t-shirts or shorts as you will spend almost all your breaks on the boat just besides the times you go on the land. To have a long sleeve shirt with you would be useful in case the weather gets cooler.
You will have many spare time for yourself on the boat. Imagine about your trip, and try to understand what you would like to do in this spare time besides swimming and sunbathing. Reading a book, listening to music, fishing and playing table games are just some of the things that you can do. Be sure you get all the needed material with you for the activities that you choose. The ones who can not live without technology, can take their computer with them and use the wi-fi facility which is available in many boats.
In addition, to have a booklet which gives information about the bays while going around will be useful to plan your holiday. Do not forget to take your camera with you by taking consideration that you will come across with breathtaking views.