"Add water and mix well" = "Ajoutez de l'eau et mélangez
bien"
"Add the water and mix well" = "Ajoutez l'eau et mélangez
bien"
The former does not sound right, as it says basically "add any
quantity of water".
The latter means "add the water" (whose quantity is mentioned
previously in the recipe,) so it seems to work better.