What did ancient greeks eat on long journeys/voyages?

They lived off the land, and often purchased foodstuffs from local folks

The Greek sailors, like those of other countries, tended to hug the coastlines of the Mediterranean Sea, so their voyages were not as long as those of the Portuguese along the coast of Africa or the Norsemen and Columbus across the Atlantic. They stopped at ports along their way and picked up supplies of fresh food, wines and fresh water. They may have left home with a good supply of lentils and dried meats and fish.

They stored grain and corn in the ships.

Not to mention all the fishing they did on route

Something akin to Jerky. Besides the Greeks as rule simply went from island to island. They very rarely were away from land for more than a day or two.

lentils

Sheep.

Moussaka and Pastitsio!

Hope that helps!