Tipping while in Dubrovnik
Every country has its own tipping rules and Croatia is particular in its own right. Tipping is expected amongst tourist workers such as local guides, waiters, bellboys, ship staff. There is no fixed percentage they expect because in Dubrovnik the gratuity is a reflection of a good service. The more satisfied you are, you show it with a more generous gratuity.
If you wish to award someone you should use cash, especially in restaurants because a lot of places don't have an option to put your gratuity on a credit card bill and if they do a big question is if a person you are tipping will get all of the amount, if any (unfortunately).