Non-negotiable
Cash. The item forgotten most often. National park fees are paid on arrival and in cash only: at Phi Phi and the Similans that is several hundred baht per person, less for children. On the islands almost nobody takes cards — not the cafes, not the shops, not the kayak rental. Carry a thousand to fifteen hundred baht in small notes on top of the fee.
Reef shoes. They cost about two hundred baht on the island and solve three problems at once: sea urchins among the rocks, hot sand and a slippery gangway. Flip-flops are unsafe on a boat, and walking barefoot over reef shallows is not something to attempt.
High-factor sunscreen, ideally free of oxybenzone and octinoxate: both harm coral and are formally banned in a number of Thai parks. Apply before departure and reapply after every swim. Light reflected off the water burns faster than it does on the beach, and sea tours are where people get scorched.
A hat and a rash guard or light long-sleeve top. There is little shade on deck, and an hour of snorkelling face down is a reliable way to blister your back. A shirt in the water works as well as cream.
Water. The operator usually supplies a bottle or two, but in heat that is not enough for the crossing. A litre per person on top is sensible.
On seasickness, specifically
The crossing to Phi Phi takes about forty-five minutes by speedboat and to the Similans about ninety, and in swell that is a real pounding. People who are fine in cars and planes still get sick.
Take the tablet thirty to sixty minutes before boarding, not once you feel unwell: after symptoms start it barely helps. Dimenhydrinate and its equivalents are sold over the counter in any island pharmacy.
What helps without tablets: a seat mid-boat towards the stern, eyes on the horizon rather than a phone, a light breakfast instead of a heavy one or none at all, fresh air. What does not help: sitting in a closed cabin, reading, and discussing how you feel.
What makes the day much easier
A dry bag, or at minimum a sealed phone pouch. Transferring to a longtail and wading ashore soaks everything in an ordinary bag. A pouch costs around a hundred baht and gets constant use at Phi Phi.
Your own mask if you take snorkelling seriously. The boat's kit works but leaks more often and is not fitted to your face. The same goes for a snorkel.
A microfibre towel: it dries fast and packs small. Hotel towels usually cannot be taken out.
A dry shirt for the ride back. Sitting wet under air conditioning at speed is genuinely cold.
A copy of your passport. Documents are checked on some routes, particularly near provincial boundaries and at the Similans. Taking the original onto the water is an unnecessary risk, but keep a copy or a phone photo.
What to leave behind
Fins — group boats almost always carry them, and a standard snorkelling stop does not need your own.
A large backpack and anything valuable. Space on board is limited and bags sit unattended while everyone is swimming. Leave laptops, jewellery and spare cash in the hotel safe.
A hard-cased selfie stick and a drone: flying is prohibited over most national park islands, and the fines are real.
One general point: skip single-use plastic. Thailand's national parks formally bar it, and staff at the entrance may ask you to leave bags and polystyrene behind.