I think this requires high precision photoshop skill by adding a Selection for the smaller parts of boat sails.
Try to be as bigger as possible in your screen to select the visible part of the sails.
My advice would be :
Blur the edges of sails (very precisely) so that it looks natural and blending to the sky.
+1 Yep, pretty much agree on this.
The higher resolution the original, the better the final result.
Also demands 'manual editing' especially on those sail masts if you want a professional looking result.
Some careful editing with the lasso tool set to feather would be my starting point - it's not that difficult, it just takes patience and time.
Also, as it stands, the resolution of the image posted posted does not contain enough information or detail on those sail masts, so you would have to guess - doable, but difficult.