Petra,
Depending on which part(s) of the image you are referring to, there are, from the top:
1) A more reddish area of sky (smooth),
1) - 2) A curved transition,
2) A more bluish area of sky (smooth),
2) - 3) A horizontal transition,
3) An even more bluish area of water (smooth/slightly textured),
3) - 4) A horizontal transition,
4) A darker bluish area of water (more textured),
4) - 5) A horizontal transition,
5) Another bluish area of water (colour like 3) and even more textured than 4)).
Any (partial of full) representation of the image that includes 1) and and 2) also includes the curved transition 1) - 2), so I am afraid your wish to use a simple gradient is unrealizable in that case.
There are other ways, but one is to create a Blend between stroked paths, both straight and curved. A safe way would be to give all of them the same direction and the same number of corresponding Anchor Points, the inner ones with Handles dragged out in the same direction, and to make sure their stacking order corresponds to their posisitons, and with a large number of (Specified) Steps. Depending on the exact shape it may be sufficient with just one inner Anchor Point on each path.