Assuming you meant by ferry/catamaran rather than private boat transfer (which would be incredibly pricey!), there are a couple of catamarans that sail from Dubrovnik to Hvar - one in the morning, one in the mid-afternoon. See details here: www.visit-croatia.co.uk/index.php/croatia-destinations/croatian-islands/hvar/getting-to-hvar/ (scroll down for options from Dubrovnik).