Horizon blending

[dsrjohnstone]

Hi all,
I've tried looking on YouTube etc but struggling to find an answer to my question.
Can anyone kindly explain how one goes about blending horizon (water & sky) as in the image below?

 

[Johan Elzenga]

Shoot when it is hazy? You could easily do this in Photoshop, so if it was done on post, then I would think it was done in Photoshop, not in Lightroom.