These guys are traditional water men of Morocco, they would carry skins filled with water around the city and provide refreshment to those who might not be able to get to the river or the fountains for a drink.
Now days they’re just a tourist thing, they still carry their water skins and you can drink from them (though obviously, if you’re a tourist, it’s wiser to just carry your own bottle of water with you) for 1 Euro. The same charge applies if you want to take a photo with them.