Is it obligatory to wash clothes bought in a European country – where dogs are prevalent – before wearing them, and does one become impure if one steps on water of unknown origin in these places?

There is no need to wash newly purchased clothes, for the default state of things is purity. A ruling of impurity is only made with certainty, and there is no certainty that the garment was touched by a dog's impurity. The same applies to the water mentioned; its default state is purity unless its impurity is confirmed. What touched the person from it does not necessitate washing it.