Water
There are natural fresh water springs outside all of the refuges. I systematically filled up with two liters of water before leaving every morning and sometimes at midday. I used a water pouch with pipe. There are often springs along the way but not always. You will see springs marked on the map with a small blue tear. On the ground, they are sometimes indicated with a sign and you will find a black pipe has been inserted to make it easier to fill up.
You should never drink directly from the rivers. It is well worth carrying a packet of water purifying tablets, in case you are on a long section where there are no fresh water springs. You will always find water that can be made safe by using a purifying tablet.
Charcuterie
Another popular starter is 'Assiette de charcuterie' or dried pork. This is also a good picnic option because it doesn't go off and its easy to transport. The charcuterie is produced locally and is one of the best in the world, the pigs are free range, they have a good diet and the meat is dried for months, so it won't go off in your rucksack.
Coppa and Lonzu - are quite different, they are made from different parts of the pig. The coppa is a combination of meat and fat, whereas the lonzu is a lot leaner.
Saucisse - pork sausage
Figatelli - liver sausage
Prizuttu - wonderful dried ham often cut with a thick border of fat
Pancetta - smoky bacon
Cheese
There is a long tradition of sheep and goat rearing on the island, there is a lot for sale and a lot incorporated in the meals on the GR20. Generally, the cheese is either brebis (ewe's milk) or chevre (goat's milk.) Then the family name of the producer is given to the cheese. You will come across brocciu, this is a whey cheese akin to ricotta. It's creamy and is often on the menu in cheese and mint omelette, or cannelloni stuffed with brochiu and mint.