Take ferry from Venice to Pula, from Pula take ferry to Zadar.
From Zadar take bus to Split and from Split ferry to Dubrovnik.
Or, you can take train or bus to Pesaro or Cesenatico in Italy and ferry from there to Mali Losinj, from Mali Losinj take ferry to Zadar. From Zadar proceed to Dubrovnik as above.
It takes a bit of time but can be very pleasant journey.
See all ferry info at croatiaferries.com website