When we leave Athens, May 2014, we want to visit Santorini and Mykonos. I cannot find ferry schedules (sites say they don't show schedules yet for May) and I want to know so I can plan better and so that the ferry isn't booked up. Also, will I need to pay in cash if I just pay at the port? How can I find out if a ferry connects Santorini with Mykonos? Or even determine which is the best way to plan out this part of the trip?
A lot of schedules will not be posted until six months out. Just days of the week for next and that will give you the schedule for next year. It could vary some but not significantly so. You can use a credit card and they have ticket offices at the port. Again use next for your scheduling info between Santorini and Mykonos.
As you noted, schedules are not yet posted, but for May, you can be assured that it would be rare that ferries sell out. There are several ferries each day from Athens (Piraeus) to Mykonos, and daily ferries from Mykonos to Naxos and on to Santorini. I suggest as your travel approaches, you check to get times, then when arriving in Athens, go to a travel agency and get your tickets. You would also be fine just to go to the port and get tickets before departure, or the day before. I would suggest though flying from Santorini back to Athens, a very long ferry ride otherwise. Try Aegean air, book well ahead (Several Months)