In many cultures, there are superstitions/rituals carried out (which have no Islamic basis) to prevent bad things from happening/to make sure something good happens, for example, in many cultures, when someone leaves to go to the airport, water is thrown on the car they're going in to ”make sure” they have a safe flight, are such rituals/superstitions a form of Shirk?

No none of our marja says it's shirk but we should follow what has been taught to us from masoomeen like giving sadqa(matching your financial status), reciting aayat al kursi, 4 qul, tasbeehat e arbaa, salwat, to leave house with wuzu