Planning to use ferries for two passengers (no car) from Venice on May 4 – to make one or two stops along the way down to Dubrovnik (need to catch a flight from Dubrovnik to Zagreb on May 7th). Notice some are overnight (which is fine) – and realize the schedule is somewhat limited in May.
Well, you don’t have too much time to travel If you have 6-7 days, that can be nice journey, especially in the summer when lot of local ferries sails around..
Anyway, I would suggest you to take ferry from Rijeka along the coast to Dubrovnik. You can make some stop-overs on the way ( Split, Hvar, Korcula or Mljet) if you wish and have time for.