Hello ,
In order to make a large open world game like this, you need to take into account all of the variables you want to be strong and apparent. This in turn send the visual message to the player you desire in order to create strong immersive environment and game. If you already have your map built then that is great, but keep in mind building your iterations with a map of this size will take a very long time unless you can split it up into sections.
The first thing you want to tackle is the textures and materials you want your island to have. For example, rocks, sand, grass, etc. The next part is filling your environment with foliage like trees, bushes, and other meshes. For the grass that will be seen when you are on the ground, you will not need to render these foliage meshes when in the air. The most important thing in your case would be to have Level Of Details for every material so when you are far away from the object it is not trying to render your assets with full resolution textures. This will save your graphics and overall processing so it can be allocated elsewhere.
As for the water, you will also want to have Levels of Detail (LOD’s), if your island will be surrounded by a plane. I actually wrote a tutorial and posted it on the Wiki on how to create a Water Shader which has also has the actual material editor set-up pasted into a notepad document. There is also a Swimmable Water Volume tutorial linked at the bottom. This should give you enough information to get started with creating your open world. Keep in mind, it takes large teams to optimize something like this and usually starting small and expanding on your core concepts can yield better results.
Water Shader Tutorial
Swimmable Water Volume
Cheers,