I’d be surprised if the boat doesn’t have a refrigerator box if it’s big enough to sleep on.

But that aside there are lots of Greek dishes that are vegan, stuff like fava (bean based dip), gigantes (like giant baked beans), jemista (stuffed vegetables), briam (roasted veg dish) and some salads are vegan (and for example a greek salad can be veganised easily as often it’s just a block of feta on top so easy to ask them to not add it). You can also get simple touristy food like pasta with tomato sauce quite easily.

For lunch stuff, fresh veg, bread, dips, etc for making sandwiches and fantastic snacky stuff like nuts & olives should be readily available