Brain freeze time. The bastard in my head is telling me 'Sea's horizon' doesn't make sense, as in: To my left the dark grey of the sea's horizon would appear and disappear with the undulation of the landscape. I know that technically it should be: To my left the dark grey of the sea on the horizon would appear and disappear with the undulation of the landscape. But it doesn't have the same flow.
Some context is required. Is the narrator on land and seeing the sea in the distance? Or is the narrator on a boat looking at the edge of the sea in the distance? I believe sea's horizon would refer more to the latter case.
On land seeing the sea in the distance. I played it safe and went with my second. 'Sea horizon / sea-horizon' may be technically correct (although I didn't know this, obviously) but I'm not keen on how they sound out of the mouth.
I have sailed for as long as I can remember and have amassed a lot of literature on the subject. I don't recall ever seeing the term sea's horizon. At sea it would simply be the horizon; on land the sea-horizon.
Good to know. But I think @OurJud is going more for poetic effect than accuracy, which is perfectly fine in my opinion.