9. We (swim) in the sea if there (not/be) so many sharks there.

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.