The Croatian currency is the Kuna (HRK), therefore it should be used to pay for all services in the country*. Occasionally it is possible to pay mooring fees in Euros, but this marina specific and should be checked in advance.
Cash is king in Croatia and you can find ATMs everywhere. Tips must be paid in cash by law. Smaller bars/cafes, restaurants, shops, and excursions often only accept hard currency. You can use Visa/MasterCard in larger establishments, gas stations and supermarkets.
* Except the crew, who are paid in Euros.