Multi-island itineraries are the classic Thai adventure, and they need a little cash planning because facilities vary hugely between islands. The groundwork starts at home: buy Thai baht in advance with a commission-free provider such as Travel FX and split it sensibly across your luggage.
Larger islands are well served, but smaller spots may have a single machine that runs dry in high season, plus fixed fees on every foreign withdrawal. Carrying a sensible float of baht keeps you moving.
Ferries, park fees, beach huts and boat taxis are almost always cash. A rough daily cash budget per island, with a reserve for the unexpected, works well.
Travel FX has supplied travel money online for more than 18 years and holds a 5-star Trustpilot rating from around 32,800 reviews.