We use US dollars. In mainland Ecuador credit cards are widely accepted but in Galapagos while most boats and hotels accept credit cards, some require payments in cash. So bring cash for the bar on board and ask your hotel operator in advance how you will be expected to pay. You may want to carry some change for small items such as pay phones, water or other personal expenses you may require. You’ll also need cash for tipping on board the ship.
It’s also a good idea to let your bank know of your travel plans and ask if there are any special fees associated with being in Galapagos.