Uncharted Waters Online(originally known as Age of the Great Voyages) is a series of Japanese privateer/trader video games set in The Cavalier Years. The first game was developed by Koei in 1991 (for PC-88, MSX and NES, and later for Sega Genesis, Super NES, and the PC). The second installment, Uncharted Waters II: New Horizons, followed in 1994 on NEC PC-98, SNES, and Genesis, later ported to PC, Sega Saturn, and PlayStation. Unfortunately, the second game was the last to be officially translated into English. Two more sequels, a Gaiden Game, and a MMORPG based in the setting never made it out of Japan. 

Gameplay-wise, the series is a Wide Open Sandbox with RPG Elements in it. You play as an owner of a small fleet (up to 5 ships in the first game, up to 10 in the second) and are free to engage in any kind of activity on the high seas: trade, gambling, ship pimping, piracy, Uncharted Waters Online Gold treasure hunting, exploration, even global politics, once you have the money (and firepower) to. What set the first game apart from its Western competitor Sid Meier's Pirates was the sheer size of the game world: instead of being confined to the Carribbean, you have the entire world to explore. The exploration gameplay was further enhanced in the second game, where you could sell the maps of your voyages, find natural and cultural wonders around the world, and look for even more treasures. The underlying gameplay mechanic involves ship management (crew, supplies, captains, repairs), character evolution (both the PC and the captains you hire), port development (trade balance, investments), and maneuvering between the major factions (Portugal, Spain, Turkey, and pirates in the first game; England, Holland, and Italy join the club in the second). It's pretty complex but not excessively so.