From A Wiki of Ice and Fire
and the location of the Summer Sea
The Summer Sea is a body of water located south of Dorne. It stretches eastward from Westeros around the southern coast of Essos and the northern coast of Sothoryos.
West of the Summer Sea is the Sunset Sea. Just east of Dorne, the Summer Sea is separated from the narrow sea to the north by the island chain known as the Stepstones. South of Westeros and the Stepstones are the Summer Islands. East of the Summer Islands are Naath, Sothoryos, and the Basilisk Isles. North of those locations are the Valyrian peninsula, the Gulf of Grief, and Slaver's Bay. The eastern Summer Sea meets the Jade Sea at the Jade Gates in the north and the Cinnamon Straits in the south.
References and Notes