I did this trip in June 2016 on board this boat.
To pay your account on board it was possible to pay by credit card or with Kuna and I think they even accepted Euros. I used Kuna to buy day to day needs when not on board, e.g. drinks, snacks, etc. The most expensive things for me were the dinners that were not included. We chose to eat in groups and the restaurant costs were similar to chain restaurants in UK.
I never found the need to get anymore Kuna in Croatia and it was easy to change back to Sterling in UK.
Hope this is of some help.