Hello!
All companies have very good coverage in the sea with both GSM and GPRS! Almost all the teritorial waters are covered! Friends who go sailing in the Aegean call me with no problems...
As for data, since you're staying long and mobile will me your main means for accessing the internet, provided that the WIND Plus non-stop solution works only for basic web browsing and chat, you might want to consider the greek providers' data card solutions with various offers. WIND ofers Data Card Non-Stop with a 20GB/month fair usage policy.
Cosmote currently offers unlimited data until September and I do think that after this offer they will re-adjust their tariffs to at least match the WIND ones, same for Vodafone, they offer unlimited data on their Vodafone Data Card programs until September and are expected to re-adjust prices after that, or even extend the offer!
Cosmote claims to have currently covered 80% of the population with 3G and does have the most extensive 3G network in Greece with optimistic unofficial statements by company insiders that they will try to have a complete (!) 3G network overlay by the end of the year!
Cosmote has by far the best 3G coverage in rural Greece (outside the main cities of Athens, Thessaloniki, Patras, Larissa etc.) so it might suit you better for mobile internet, if your needs exceed typical low-speed browsing that you can easily do with WIND's GPRS and their inexpensive Plus non-stop offer (3,49€/month compared to more than 20-50€ month for data-card plans).
Cosmote's extensive 3G network though is said to be lacking the necessary backbone, thus the 3G network is "virtual" in some places, meaning that although you have 3G speeds from your mobile to the base station, the speed is limited on the station's connection to the company's network, thus you get slow 2G-like speeds...
WIND on the other side has overlaid its network with EDGE since 2004 but has never offered it to its customers for unknown reasons, it is not known if they're ever going to offer it after all...
Look at the providers' sites for their data-card offers to see if there is one to suit your needs. All sites are available both in English and Greek, so you won't have problems finding your way around. It's
www.cosmote.gr,
www.vodafone.gr and
www.wind.com.gr (not
www.wind.gr since some clever guy has taken the domain name before WIND rebranded)!
Notice though that in the last months, mobile data prices are going down due to renewed competition between the companies so prices are expected to fall in the coming months...
For example, the cheapest DC offer right now is offered to university students only and it offers unlimited (20GB/month fair usage) access from 20:00-08:00 everyday and a mere 70MB/month from 08:00-20:00.
This is a quite new offer introduced only two weeks ago, so it is expected that the student offer will be matched by the other companies too and that similar inexpensive unlimited programs (maybe with time restrictions like the afternoon program mentioned above) are expected also for the non-academic community!