As of last week there was no replacement for the Neptune. So there would not be an 8:30 departure from Ag Roumeli. The earliest ferry would be (currently) the Samaria, coming from Paleohora/Sougia around 10:30. It also does not appear to be holding to the 11:30 departure, sometimes leaving earlier. But ANENDYK were still struggling with their improvised timetable, needing to work in the crossings to Gavdos and cover some of Neptune's route. For example, the Sfakia/Loutro 'shuttle' that Neptune did was dropped.
I can imagine that this situation can change week to week. My experience with ferry travel everywhere in Greece has been to not make fixed plans assuming a 100% guaranteed time table. Check the situation when you are there, and make plans accordingly .. keeping alert for the weather situation as well.