A global pass is valid in Italy and on ferries from Italy to Greece (some ferries only give a reduction).

For domestic ferries to Greek islands, some ferry companies give a reduction and others don't.

Please check:

• Pass benefits in Greece
• Ferries in Europe

If you only get a reduction on the ferry fare, then you don't need to use a travel day.

Just a side note ontop of the great suggestion of @rvdborgt. Even the included Ferries charge some addional fee´s like: FuelSurchargem, Porttax or seasonal surcharge. :)