I went on the Adriatic Coastal Explorer last September - similar itinerary, I think, but different ship. I can't remember how much I spent - I don't buy postcards and only one or two very small souvenirs for myself but did have some meals out and of course left tips for the crew and TM - but my account book shows that I originally bought £250-worth of kuna in the UK (I don't have a note of how much, if any, I cashed back in afterwards). However, I did take my Euro currency card with me to use in ATMs if necessary. Apart from one very small port all the other places we called at had one or more ATMs, sometimes tucked away in a side street. In the big ports there were also currency exchange shops which gave a good rate (better than in the UK, in fact). If you don't want to take a lot of kuna with you, you can either use a Euro currency card in an ATM or change euros or pounds at an exchange as long as you plan ahead so as not to "run out" at or on the way to the very small port (whose name I have forgotten).
I hope this will help. Have a lovely holiday - I thoroughly enjoyed mine - but don't forget to pack some travel sickness tablets in case of rough weather.
Ann