Pig Island is a short, gentle trip, and that is exactly why people underpack for it. The crossing is fifteen minutes, the beach is a beach, how hard can it be. Then the boat lands through the water, the spray catches the bag, the bar only takes cash and the reef is shallower than the sunscreen bottle assumed. Here is the list that covers all of that, learned from watching guests board every morning.
Swimwear goes on under light clothes at the hotel, because there is nowhere to change on a speedboat and not much more on a longtail. Sandals or water shoes with a heel strap handle the wet landing; loose flip-flops end up in the sea or in your hand. A hat that survives wind, sunglasses, and a rash vest or long sleeves if you burn, because the open boats give no shade on the crossing and the beach gives very little at midday.
Cash first: THB 50 per adult for the landing fee on every tour, plus money for drinks and for pig feed if the bar is selling it that day. Our fee page explains who collects it and why it is not in the ticket. Then reef-safe sunscreen, because the Koh Tan reef sits close to the surface and ordinary formulas harm it. A dry bag or a zip-lock for the phone, since every boat but the catamaran takes spray. A towel. And your own mask if you have one that fits, because the boat's masks are fine but a good seal is personal.
Motion sickness tablets, taken before boarding rather than when the horizon starts moving. The crossing is short and sheltered, so most people never think about it, but if boats have bothered you before, the catamaran is the flattest ride and the calm months from February to August give the smoothest sea. Any medicine you rely on comes too. Small snacks for children help with the gap between an early pickup and lunch at the pier.
Food in your pockets, because the pigs will find it and the bites people report online nearly all start there. Passports, jewellery and anything that cannot get wet or sandy. Drones, unless the crew has agreed in advance. And the full snorkel set: mask, snorkel and life vest are on every boat, and the reef is shallow enough that fins are more nuisance than help.
That is the whole list. Pack it the night before, book the boat that suits the people you are with from the comparison page, and the morning takes care of itself.