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. If all goes well, it takes about a month from San Augustin (the most southwestern port in Theah) to reach Marcina, the Castillian colony which is the de facto capital of the Archipelago. Because of favorable winds, it also takes only about a month to reach the Straits of Blood, a notorious pirate port in a string of islands east of the main chain. Returning from the archipelago can take almost twice as much time as getting there, as the trade winds run east to west.
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.
Sea Travel
Sailing ships have a maximum speed of 75 to 100 miles per day under favorable winds and currents, but most voyages west have periods without such favorable winds - particularly in a region called the doldrums, where winds often seem either non-existent or are instead whipped into great storms. The general direction of winds - the trade winds - brings ships from east to west easily, but tends to make return voyages lengthier.