City's first goal was without doubt controversial but this stems from ambiguity in the offside law. This is because there have been so many directives and reinterpretations of it that referees must be dizzy trying to make a decision because of the use of words such as "deliberate". What was Janmaat trying to do "deliberately" when he played the ball? We saw a similar case in the romp against Chelsea. Barclay headed the ball. He headed it "deliberately" though I doubt he tried to head it "deliberately" to Aguero. Aguero was in an offside position when Barclay acted and he was in that position "deliberately" waiting for Barclay's error. When Sergio stuck the ball in the net no-one questioned the legitimacy of the goal. So, did Janmaat play the ball "deliberately"? Well, certainly, seeing as he slid a few yards to do so. Did he intend to play the cannon off Sterling and into the net? Obviously not. Was Sterling offside when Janmaat played the ball? Yes, clearly. Was he played onside? This is where the ref decided that IN HIS OPINION he was. It's certainly not an opinion which is held universally but it is based on a permissible interpretation of the offside law.
As for your contention that it was not THE turning point of the game.... we'll never know, but goals do change games. Our second (and third) followed so hot on the heels of the first that claims that Watford were in disarray for a time cannot be dismissed lightly. But as a City fan I have to say that a goal for City was nowhere near as unexpected or hard to imagine as is now being cliamed in the Sunday papers. City were a model of patience and, as the games with WHU and Bournemouth showed, as the whole season has shown, keeping City at bay for 90 minutes is no easy task at all. Wit that I'm in total agreement.