It's also worth noting in this age of fluctuating prices for everything which makes people think they will save money by booking ahead, that Greek ferry prices are fixed by law. So there is no difference between buying a ticket ahead of time or on the day of departure. The only small risk is if the ticket class you want is sold out. ie. private cabin. But that is not a high risk at all.
Besides, if you really want to go 'island hopping' which it doesn't sound like you really are given your list, but nevertheless, you should be going 'deck class' with the real 'island hoppers'. It is literally what the name says. No cabin, no bed, just a seat inside somewhere perhaps or a night spent in a sleeping bag laid out on the deck. For the more 'savvy' island hoppers, you get on board as soon as you can and stake out a space under a stairwell for the night. Inside out of the wind and possible rain, on a carpeted (perhaps) floor. Ah the sheer luxury of it compared to the non-savvy deck class passengers. They'll be green with envy.
