Combine flour and paprika in a bowl, then dust sardines in flour, shaking off excess. Heat half the oil in a large frying pan over medium-high heat. Fry sardines skin-side down until crisp and golden (2-3 minutes). Turn and fry until cooked through (1-2 minutes). Transfer to a non-reactive container.
Combine vinegar, currants, onion, sugar and 100ml water in a saucepan over medium-high heat, bring to a simmer, then pour mixture over sardines and set aside for 15 minutes.
Heat remaining oil in a frying pan over medium-high heat, add garlic, rosemary and bread and cook until golden all over (2-3 minutes). Drain on absorbent paper (discard garlic).
Combine watercress, parsley and oranges in a bowl, add sardines, 2 tbsp vinegar mixture and olive oil, toss carefully, season to taste and serve.