Hi Philipp,
I know for sure the ferries from Athens to Kalymnos continue year-round approximately 3-4 times per week. So does the Rhodes – Kalymnos ferry, although I don’t know how often per week. I looked up the timetables also, but they are only available through October 31st. That’s the main problem, that timetables become available only about 1 month before.
I emailed both the Athens – Kalymnos and the Rhodes – Kalymnos ferry asking them about November, and if I get a reply I will post it here. They have been good about replying in the past. The problem is that I’ll be offline for a few days (Monday to Friday) and won’t have an answer until then, so perhaps you’d like to email them as well? The websites are:
Athens – Kalymnos (‘Diagoras’ ferry)
http://www.bluestarferries.gr/site/content.asp?sel=616&loc=2&query=
Rhodes – Kalymnos (‘Dodekanisos express’ and ‘Dodekanisos pride’ ferries)
http://www.12ne.gr/en/
There is an English version on both sites.
We always take the ferry from Athens to Kalymnos in winter and there’s never been a problem, severe weather is uncommon.
I hope this helps, feel free to post more questions if you have them!
Katie