Is it possible?
It looks good - waves built by the ocean modifier - but it really taks an awful lot of time to render -
Is it possible somehow to “prerender” it, somehow? so that Blender´s internal render engine doesnt need to start from scratch each time I render a scene.
I have characters and other models in the scene that needs to be redone etc and rerendered several times to check out how they look etc. but the ocean is just perfect and doesn´t need to be changed - yet its the ocean that “destroys” the render experience 
Is it possible somehow to “prerender” it, somehow? so that Blender´s internal render engine doesnt need to start from scratch each time I render a scene.
Some options
Make the ocean object not renderable (camera icon in outliner)
Put the ocean object in its own render scene, disable when not needed
Render out ocean scene to an image sequence, add back to your scene as an Image input node in the compositor