cheapest ferry company and/or cheapest budget airline to get from athens to

Ferry: Blue Star Ferry or Aegeon Pelagos Sea Lines Airlines: Aegean Airlines or Olympic Air

what are air seats and how do they differ from other seats? would they be ok to sleep on in an overnight ferry/

Air seats are similar to the seats on an airplane. They recline for comfort and yes, you can sleep in one of them. They are also reserved, so for just a few euro more than the cheapest economy ticket they are worth the extra money. No one can steal your seat if you get up and go for a walk, which can sometimes happen with an economy ticket.