Oil is needed for marinades when you are using spices, as most spices are oils. The oil added to the marinade provides a medium for flavors like cumin and chili, helping liberate them from whatever powder or whole spice they are in. Water doesn't do anywhere near the job that oil does on these, you're better off just rubbing them on directly.
Oil is also sticky, so it will cling to the meat after it is removed from the marinade, keeping some of the flavor outside.
So if you are making a marinade that uses powdered or whole spices you need oil to best extract them, and there is no substitute. You are better off choosing a marinade that uses herbs, acids, water, and salt.