Cruise from Venice to Athens(Pireaus).

Not to Athens, I don't think. Anek-Superfast Line runs a ferry from Venice to Patras, Greece, on the Peloponnese, for instance departing at noon one day and arriving at 9 p.m. the following day. I don't expect that the ferry travels very close to the Croatian coast and it's lovely views. From Rome to Athens, there are several nonstop flights.

Karen,
If you are looking for a cruise vs. a ferry, then yes, several cruise lines offer 7-day cruises from Venice to Athens.

One you might want to check out is Seabourn, which is a high-end small ship (fewer than 500 guests), all suite, all-inclusive ship (except shore excursions). Right now, some of their sailing dates offer the cruise for as little as $2999 on select dates.

We have traveled with Seabourn before, and we found it a lovely experience.

I also just looked at Silversea's site. Silversea is also a high-end small ship (also lovely experience), and they have a Venice/Athens sailing that is waitlist only....did not find the price.

The larger ships may also offer a similar itinerary. Check out www.cruisecritic.com to see which cruise lines sail in and out of Venice/Athens.

If your have a very generous budget, SeaCloud, the vintage sailing ship (which was a wedding gift from E. F. Hutton to Merriweather Post (of the Post Cereal fortune) offers sailings between Venice/Athens.