How low can the sun be seen in the sky?
The lowest exposed point on land is the dead sea shores, at -413metres below sea level. so theres a good chance you could find a spot that lines up correctly to give you a view of a piece of the horizon well under sea level. I dont believe that elevating yourself is the key factor here though. At Around 35000 ft you can start noticing the curvature of the Earth, the horizon being 200 some odd miles away (Pythagorean theory -theorem?- can be used for this, several sources on google explain the steps to find your equation, and far better than I can. ) but the distance to the horizon grows rapidly. So you would certainly  want to find the sweet spot of height /distance, too high and you wont know when the land has ascended back to sea level, as the terrain will become more vast and harder to judge detail or distance. Too close to the shore and your only seeing a few miles but the angle of sight wouldnt be the best possible for finding the lowest horizon. You would want to work out the math and find a spot that puts the dead sea shores as your visible horizon, bc if your standing there, watching the sun rise, the sun is rising over a horizon that isnt the lowest accessible location...