Travel from Athenas to Istanbul

BUS-probably a very few, maybe not daily, direct (that means no change, not NOT no stops).

NO ferry to speak of-there are a very few and always very expensive short trip ferries from GR isles close by to TR to nearby TR port, mostly aiming daytrippers.

Train: there are long gaps and due to that will take much more time-many changes-as compare to direct bus. In SUMMer-often to Thessaloniki-a very few onward till some stop 30/40 KMs before border with BG (I always forgtta name of that stop-read seat61.com for the info)-bridge gap to KUlata=first borderstop in BG-old dilapidated train toSofia-then 1/day Overnite train -couchette- to IST.

This will cost you 2 nites and the day in between.