yes zdzichu, the ferry schedules for winter-time services are very different to those of the summer.

Also, those services, fewer in number, are possibly more likely to be subject to cancellation due to the weather. Don't let me put you off though as many islands in winter have a special kind of magic and if you find a ferry that operates then go for it. If I can give any advice it is to allow yourself a day or so on either side of your trip 'just in case' of delay.

This is a good site for Greek ferry winter timetables (use the 'ferry schedules -domestic section') :-

http://www.gtp.gr/

Bill

That particular article is way out of date. One of the ferry routes has changed the Piraeus > Samos ferries now call in at Syros/Mykonos not Paros/Naxos.

The main ferry routes are the same in winter. A lot of the cross routes are less or dissappear in the winter but main routes are the same.

The problem is knowing the ferry routes, I know them, I've been Greek island hopping for 25 years. There are dozens of routes to around 100 islands, so I can't detail them here. The only websites that shows some of them is the Piraeus port website showing ferry and ports of call.
http://info.yen.gr/page_en.php?id=1617

The guide book 'Greek Island Hopping' pub Thomas Cook, is invaluable for showing you possible ferry routes and Islands.
http://www.greekislandhopping.com/Home/Homefr.htm

GTP.GR is for experienced ferry users, but tricky for a newcomer to Greek Islands.

Even your presumption of ferries to Crete and Rhodes is wrong. Ferries go direct to Crete, no stops. Ferries to Rhodes stop at a number of less well known Dodecanese islands on the way, like Patmos/Leros/Kalymnos/Kos then Rhodes. So you can island hop on ferries to Rhodes, but not to Crete.

The ferries schedules are very different in summer and winter.
More, be aware that online ferry systems like greekferries.gr and ferriesingreece.com have not schedules for this summer. But it is not their fault. The Greek ferry system is bad and ferry companies announce their schedules just a couple of months before summer!