2 sentences

1. 海盗在公海上横行，威胁着商船的安全。

   hǎi​dào zài gōng​hǎi shàng héng​xíng, wēi​xié zhe shāng​chuán de ān​quán.

   Pirates run rampant on the high seas, threatening the safety of merchant ships.

   Source: Dong Chinese Dictionary

2. 这艘强大的战舰正在公海上执行巡航任务。

   zhè sōu qiáng​dà de zhàn​jiàn zhèng​zài gōng​hǎi shàng zhí​xíng xún​háng rèn​wu.

   This powerful warship is carrying out a patrol mission on the high seas.

   Source: Dong Chinese Dictionary