Watford faced City at the wrong time of the season while they're fighting to save their own lives. The scoreline wasn't surprising.
City are definitely favorites for the UCL. Their hunger may be the only distinguishing factor between them and Real Madrid, but the Spanish giants won't go down without a fight.
Poor Watford, City has crushed them on many occasions in recent years, I think that after today's game they will have many things to correct for the second leg, it seems to me that they lacked firmness in defense and in Madrid they cannot afford this.