9. We would have swum in the sea if there hadn't been so many sharks there.
В условных предложениях третьего типа (Type 3) в придаточном предложении (if-clause) используется Past Perfect, а в главном предложении (main clause) используется would have + Past Participle.
Ответ: We would have swum in the sea if there hadn't been so many sharks there.