Maybe you should split it up in subgames, one that goes into great detail about sailing, with very intuitive guides, for example how to sail against the wind, the other part more of a game that allows to explore the landscape. However as soon as you want to compete with todays graphics in 3D rendering, you require a huge world that can be streamed, or a very good on-demand procedural environment generator. So, the style of the world is an important decision to make, as it quickly reaches the limits, in rendering capabilities, as well as amount of work in content creation.