Open Sea
noun
The open sea is where the sea joins together to be the open sea.
The readings are the kun'yomi readings that you learned when you learned the kanji. Just remember that this time 合 takes the あい reading.
ピンク色の怪獣は海岸から約8km沖合にいるとの報告を受けました。
They reported that the pink monster is about eight kilometers offshore.