Is there some kind of economic ticket to travel several times on the Greek ferries to the Greek islands ?
This might not be the answer you expect but Interrail passes offer some discounts on some domestic ferry lines.
The offer is you get 30% off on domestic ferry tickets if you reserved a ferry ticket between Greece and Italy (which is included with the Global Interrail Pass or you have 30% off on it with the Greece Plus Interrail Pass).
It is valid on the ferry operators Blue Star/Superfast Ferries and Minoan Lines (if all your trips are with the same operator).
So this is mostly interesting if you already travel by train (Global Passes start at 181 euros and Greece Plus Passes start at 81 euros).
NOTE: as written on Interrail website, as of June 2013, "Greek national railway company OSE has suspended the circulation of all international trains indefinitely."