Bring more than one way to pay.
A practical approach is to have access to Japanese yen as well as a payment card. Ask your accommodation and any activity or rental provider which methods they accept. A booking made online does not by itself explain how meals, extras or the remaining balance will be paid.
Before a day away from your base, think through the expenses you may encounter: transport, lunch, bicycle hire or a small purchase. Preparing for those amounts gives you more choice about where to stop. Card and mobile-payment acceptance should be checked with the business, especially if it is your only way to pay.
Check the ATM and your own card.
Japan Post Bank provides an international ATM service for supported cards issued overseas. Its official guidance explains the supported card networks and the conditions of use. It also notes that service hours and access vary by ATM, and that a particular card may not work even during service hours.
Check overseas use with your card issuer, including any fees or limits that apply to your account. Read the options and charges shown by the ATM before accepting a withdrawal. Use the bank’s current guidance and information from your own card issuer for these details.
Sort out everyday spending before leaving your base.
Before travelling to another island, check the cash and payment arrangements you will need there. Ask your host or provider about access near the places you are actually visiting. An ATM may not be close to the port or open when you arrive.
Allow for the possibility that your stay or route changes. The sea and air connections that carry visitors also support everyday life, and weather can interrupt them. Keep enough money available for an additional meal, journey or night.
A few checks make spending straightforward.
Before you travel, confirm that your card can be used abroad and keep a way to contact its issuer. Before arrival, confirm payment and cancellation terms for your stay. Before a smaller-island visit, check the arrangements for the particular businesses you will use.
Then leave room for the enjoyable part: a market purchase, a meal you had not planned or a piece made by an island craftsperson. Small businesses often have different payment arrangements from large hotels, so carry a suitable alternative.