also: そとうみ
noun
Refers to the sea outside a bay, harbor, or inland sea; the open ocean as opposed to coastal or enclosed waters.
台風が近づくと、外海は大荒れになる。
When a typhoon approaches, the open sea gets very rough.
この港から外海に出るには
To reach the open sea from this port, you must pass through a narrow channel.
Compound of 外 (outside) and 海 (sea). The reading がいかい is the on'yomi, while そとうみ is the kun'yomi reading.