As has been mentioned, there are many cafes and restaurants that have free wifi
so Skype will work for the price
of a few coffees/beers. If you buy a high gain usb wifi antenna
for your PC you can often continue using the wifi from your boat, its for you to decide if that's ethical. If most of your calls will be to outside Europe e.g. back home, then Skype will work out cheapest.
You can pay as you go and sign up to wifi adhoc and if you want to do some intensive data/skyping that can work out fairly cheap
. If you get an app for your PC/phone/tablet that does wireless tethering, then several users can share the connection as most adhoc services only allow 1 user. Again, a high gain usb wifi antenna
will help get you the most reliable and fastest connection
There are also internet
cafes as in most countries.
The EU has capped the price
of mobile phone calls and data roaming, so an EU sim with data roaming will give predictable costs but only in EU. Calls made to outside the EU will still be expensive unless you Skype.
If you stay in one country for longish periods, then a local sim will probably work out cheaper if you need lots of data. Again calls made to outside the country/EU will be expensive except Skype.