also: そとうみ
Refers to the sea outside a bay, harbor, or inland sea; the open ocean as opposed to coastal or enclosed waters.
When a typhoon approaches, the open sea gets very rough.
To reach the open sea from this port, you must pass through a narrow channel.
外界 (outside world) and 外海 (open sea) are pronounced identically but refer to completely different things.
Standard kanji spelling for the reading がいかい.
Alternate reading そとうみ is also used, though がいかい is more common.
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.