Searcing by Port of DepartureHere is how it works. Say you want to take a Greek cruise for the first week in October. You can find out what cruises are available by checking "Greek Ports of Departure" - which for the purposes of Cruise Timetables are Corfu; Iraklion, Greece; Lavrion, Greece; Limassol, Cyprus; and Piraeus the port of Athens, Greece. (We'll leave aside the fact that Cyprus is not actually part of Greece, but is an island nation - two by some counts - in its own right.)
Clicking on the big one, Piraeus Athens Greece, you'll get a linked calendar list of all cruise ships scheduled to depart Athens. This is not just on one date, but for the rest of the year and the following year as well. Since most cruise lines plan their schedules 18-24 months in advance, at least, virtually every cruise is included. There may be some last minute changes, deletions, or additions, but it gives you a good grasp of potential cruise lines operating around the date you choose.
If a precise date is important, you can just click on it. My test date was October 4th, and on that day, only one cruise is departing from Athens - Princess Cruises. In this case, it lists the actual scheduled ship, some sample prices, and provides me with a link directly to Princess. Since I'm thorough (and doubtful!) by nature, I will probably go directly to Princess another way just to double-check pricing, and I'll also check separately to see if I can find any mentions of discounts or last-minute pricing for that cruise.
If Princess Cruises is not my favorite line, or I don't like the pricing, it's easy to use the linked arrow buttons to check the day before or the day after, or even to click day by day through an entire month - though be aware some dates will be skipped when there are scheduled departures. When I click on October 5th, I have new options - a short Greek island cruise from budget line Louis Cruises and a longer deluxe line Seabourn.
Remember, this method just shows Greek cruises which originate at Piraeus, Iraklion, Corfu, Lavrion or Limassol. It won't show cruises which begin outside of Greece. To find those, go on to the next section.
Searching By Cruise DestinationYou can also search by destinations - so if you only want to see cruises which go to Mykonos, for example, you can scroll down on the Greece page until you find the port that interests you, and then check the schedule for only that port on the days around the time you'd like to be cruising in Greece. This method shows all cruises, not just those originating in Greece and it will also show cruises which may only make one or two stops at Greek ports, so be cautious before you rush to book it.
Select a Greek port which interests you, and then choose one of the dates shown in the calendar to the right.
Fun Greek Cruise Countdown TickerOnce you've found a cruise you like, you can also create a custom "Cruise Countdown Ticker" which will keep track of just how long you have to wait until you're finally where you belong - on a cruise ship in Greece.
Wondering if pricy tours booked directly through the cruise line are worth it? You may find this article helpful: From the Ship Tours in Greece - Are you taking them or are they taking you?
Before You Buy a Cruise of Greece - Find out what you need to know before choosing your cruise or tour of Greece and the Greek islands.
More on Cruises from About.com Cruises.