Islands
The 7th Sea Player's Guide provides little description of the islands of the Great Sea, only saying that there are some. Details on them are otherwise scattered throughout the books.
Three major island chains - or rather, two island chains and a singular island - are common destinations of Thean sailors heading west. In addition to these three locations, the Great Sea is studded with smaller, not-yet charted islands. Islands tend to fall into one of two bands; either they are relatively close to Theah, within 200-400 miles of the coast, or else they are some 1500 miles away along a volcanic ridge in the Great Western Ocean. While some few islands appear in the sea between, largely it is endless ocean.
Sailors believe that beyond the volcanic ridge 1500 miles away lie even more islands, but some force out there seems to make ships disappear - the Explorer's Society mounts expeditions west from the Midnight Archipelago (see below), but many of those expeditions - like Cristobal Gallegos' doomed 1402 attempt to circumnavigate the globe - never return. Some islands "West of Midnight" have been charted and reported, but journeys to them are rare and fraught with peril.
Islands and Island Chains
Midnight Archipelago -- The preeminent chain of western islands and the only islands inhabited by humans prior to their discovery by explorers, the Midnight Archipelago lies 1500 miles west-southwest of Castille and while the archipelago is only incompletely mapped it is perhaps an eight-hundred mile long snake from north to south. It takes about three weeks sail from San Teodoro (the most southwestern port in Theah) to reach the northernmost portion of the chain, and about a month to reach to Marcina, the Castillian colony which is the de facto capital of the Archipelago. It takes about five weeks to reach the Straits of Blood, a notorious pirate port in a string of islands southeast of the main chain.
Marcina is the largest colony in the archipelago, but many of the nations of Theah have small colonies in the islands - the chain has Syrneth ruins, exotic hardwoods and spices, pearls, and other treasures. Castille has discovered gold and silver in several of its colonies, with the largest gold mine on Marcina itself. Several times a year, the colonies send their treasures to Marcina where it is loaded aboard the semi-annual flota - a treasure fleet - that then sails back to Castille under the watchful eyes of the Castillian Armada.
Thalusian Isles -- A chain of eleven islands running diagonally northwest over a thousand miles west of Avalon, these temperate-to-cold islands are uninhabited save for magnificent Syrneth ruins studded with traps and other dangers. They are about two weeks' sail from Luthon, and are mostly populated by adventurers - the Explorers are the most legitimate presence on the islands, maintaining digs on three of them, while pirates and other ne'er-do-wells sometimes seek out treasure in the Syrneth ruins of other islands. The ruins are exceedingly dangerous, however, and so raiding Explorer digs is often safer than seeking out artifacts on one's own.
Isle of Syrne -- Once a pirate hideaway, the Isle of Syrne is a subtropical island three hundred miles west of Castille, about three days straight sailing. In addition to rumors of buried treasure - the patron saint of pirates, Captain Rogers, is said to have called Syrne home - it is covered in ruins worked by the Explorer's Society.