I think you are worrying too mech, especially as you don't have a connecting flight.

If A3 cancel through irrops it will probably be too late to get you on an earlier flight. In any case, JMK flights are very busy. You don't say what time of year the flight is, but I guess it's quite soon. In that case there would be 150+ others also needing rebooking and you probably end up on one of the first couple of flights the next day. In reality, ATH being the A3 hub, I'm sure they'd find a spare aircraft and operate the flight late to avoid that very substantial cost. If it were cancelled, you'd be entitled to hotel accommodation and meals in Mykonos also and the standard EU261 compensation of 250 Euro per pax. As long as you call the hotel in Athens I guess they would hold the room and the cost aught to be covered by the compensation.

Ferry Schedules are available online but I'm not sure if there is always a late evening one to Piraeus or Rafina. Looking at schedules for tomorrow (27th August), there is a 19:35 departure to Piraeus arriving 23:55. There are ones at 18:30 and 18:40 to Rafina as well as a 22:30 'redeye'. Bear in mind that ferries can be totally fully booked days in advance in August (although it's not so likely now we're over the peak). Those times won't be the same every day of course. If you chose to take a ferry instead you'd be entitled to a full refund of your air ticket as well as the compensation, but not the Hotel etc.

https://travel.viva.gr/en/ferries/bo...false&hv=false

Really though, don't worry. Take a look on somewhere like flightradar24 and try to find a cancelled flight from JMK-ATH. I bet there aren't any in recent months.

EDIT: I checked and OA379 hasn't been cancelled in the last three months. It's often a few minutes late and in the middle of August was over an hour late six days on the run! I think this is good in a way - it shows that they manage to operate the flight somehow, eventually, even if the scheduled aircraft isn't in the right place.