food or snack bar on Naxos to Pireaus ferry?

Yes. Everything you would imagine from a snack bar: muffins, sandwiches, fruit, drinks, coffee ... The ferry is very comfortable. There are several mounted TV's and when we traveled, the World Cup was playing ... not that tv is a good thing, but depending on your taste, I mention it (in our case, we like watching the World Cup so for 4 hours on the ferry, it was a good thing)

Yes, if you are talking the larger ferries, but my recollection is that the quality is pretty minimal, basically "heat and serve" stuff., bad cafeteria food.

My preference for a longer trip (ferry, bus, train) has always been to shop for my own food and drink and bring it along.

Skip buying food on the ferry as they are over priced and not really that great.

Pick up something in Athens/Piraeus like fruit or Spanikopita or other baked goods. Cheaper and far more delicious!

I've only travelled on the large ferries and there are multiple food bars. On one trip they were out of half of the options. Many people packed their own food too.