Just google tipping in Croatia, although you won't get a definitive answer. Tipping is not expected on the level that Americans commonly do. In restaurants, if good service is had (but remember there is a difference between what Amricans perceive as good service and what many Europeans call good service) 10% is welcomed. There is no necessity to tip for a drink at a bar.
This doesn't help with tipping your charter captain however, and Eva's suggestion of putting the tip in an envelope, is sensible.
A slight side note that is a little quirky: Croatians see it as rude to leave any food on the plate, so if the captain cooks for you, try to eat it all!