Short list. The sea is warm, the sun is not subtle, and there is nowhere to buy anything between coves.
Bring
Reef-safe sunscreen, factor 30 or higher, and more than you think — you burn on a boat in ways you do not on a beach, because of reflected light and wind that hides the heat. A hat that survives 25 knots of apparent wind. Sunglasses. A towel each. A light layer for the ride home, which is cooler than the middle of the day. Shoes you can walk a stone waterfront in, if we are stopping for lunch.
Already aboard
Snorkelling masks for every guest, life jackets in all sizes including children's, a bimini top for shade, cold bottled water, and a fridge you are welcome to use. You do not need to bring any of it.
The three most-forgotten things
Cash, in small notes — some island konobas still prefer it and card machines fail. Motion-sickness tablets if you are prone, taken 45 minutes before departure rather than when you start to feel it. And a dry bag or a zip-lock for phones and passports: nothing sinks a good day faster than a wet phone.
What to leave behind
Hard suitcases, glass bottles, and anything you would be upset to lose overboard. If you want photographs from the water, a cheap floating phone strap is the best five euros you will spend.