We are landing in Venice and taking a cruise to Greece and back to Venice. Do we need a multiple-entry visa or will a single-entry be enough?
That depends on the cruise.
Both Italy (where Venice is) and Greece are members of Schengen, and the single/multiple-entry thing goes for the Schengen area, so if you only enter the Schengen area once, you only need a single-entry visa. But if the cruise includes a stop in one of the non-members of Schengen that you'll pass (e.g. Albania), you'll technically enter Schengen multiple times and need a multiple entry visa. Without having done any work to verify it, my guess would be that the companies arranging that kind of cruise would either be quite explicit about leaving Schengen (meaning every passenger would need a multiple entry visa) or (more likely in my mind) would stay inside Schengen.
It looks like most if not all cruises between Venice and Greece stop at one or more ports outside of the Schengen area (Kotor, Dubrovnik, Split...).
This theoretically means there should be exit/entry checks, and possibly the need for a multiple entry visa, but there are lots of exceptions (mostly based on where passengers can embark/disembark), and only the cruise operator will really know.
There does not seem to be much information on the topic on the relevant websites I checked, so you'll probably have to call them to be sure.
To be on the safe side, I would personally request a multiple-entry visa whatever they tell you. You'll definitely need one if you actually go ashore in one of the non-Schengen ports.
You probably only need a single entry visa, but it depends on the exact arrangements between the cruise line and the port and immigration authorities. Only your cruise line can give a definitive answer.
Sea ports are in most cases considered external Schengen borders except if you use a regular ferry connection directly between two member states. Private boats going between Italy and Greece are for example always subject to immigration control and cruises are usually not considered as as regular ferry connections. There are however many exceptions in force to simplify cruise tourism, but as I wrote, you must check with your cruise line to see if they apply in your case.