In reply to dsauerbrun:
I work remotely in Europe year round (IT). I spent a few weeks in kalymnos last year ( before Europe--wide free roaming came in), I found a company that shipped a sim to me in kalymnos. I've been racking my brain for the name, but it won't come, sorry. They were based in Cyprus.
Since free roaming I've been using a mix of UK sims and Spanish Sims in france, Spain Sardinia and greece (lionidio) my experience is that foreign Sims don't get the same coverage as native ones do (Spanish Voda Sim gets 4g everywhere, UK voda/3 get hspa mostly. They are tied to specific roaming networks these days as well, so you have to get lucky with the sim you pick)
If you can get a UK sim you will probably get 3g/hspa in kalymnos (massouri, anyway), and as long as you aren't video conferencing that will probably be ok. Don't bother with WiFi, its mostly rubbish.
Have fun!