Such a pass does not exist. There is either InterRail=for trains (not very useful to travel in GR due to very limited remaining ´net´work and NO INternat connections at all OR there is GReek Islands Ferry Pass-for 4 or 6 trips, that for 6 includes also the Italy ferry. There are scores of different ferries doing It-GR and most give small discounts to ju/se-nior or passholders,

ONLY valid on a very few companies that may not even serve the islands you have set your mind on-or maybe you are so flexible to be able to change.

Just google ´name of pass’and an article will come (from ´19) that gives a very good overview of pro+cons. Google as you may know would already have given the answer on your Q right away

Please check the Greek Islands pass page and the Greek Islands Pass brochure.