The narrow sea is the sea the sits between the continent of Westeros and the eastern continent of Essos. The sea is relatively narrow and easy to cross, but large enough to make large-scale invasions and military incursions between the Seven Kingdoms and the Free Cities difficult.
The narrow sea can be rough in autumn, and autumn storms are common. In winter, the storms are worse but less frequent. It’s suggested that a journey from the Shield Islands to the far side of the narrow sea is so hazardous in autumn that two-thirds of a fleet might be lost in the attempt Thus Trading galleys some times better of crossing in the safety of the Stepstones.
It connects the Shivering Sea sea in the north, Sea of Dorne in the west, Sea of Myrth in the east and the Summer Sea in the south, which it’s separated from by the island chain known as the Stepstones. Bay of Crabs, Blackwater Bay and Shipbreaker Bay are all inlets of this sea.