Many thanks for your answer. Now that the image is rendered, let me show the result :
As you can see, the reflection is very bad against this huge sun.
I tried to increase the min highlight spread but it seems to darken more the ocean (I didn't used the reflection shader, just the standard water). I'm not that far advanced in Terragen to do better for the moment.
I post worked the image to correct some clouds against the sun (clouds are not transparent against the sun), and added a reflection in Photoshop. I'm not a genius in postprocessing, but I tried my best... Hope this kind of image will be possible in Terragen
Just love the sunsets...