I simply observe the horizon rising with me (or the optical instrument).
Would you care to show us an example of the horizon rising with an optical instrument? Preferably one with a reference level.


What is tilting the camera up or down supposed to prove?

Photographers can put the horizon anywhere they want. That's why I asked for a reference level.
Would you care to explain why things that are farther away at sea level appear higher than things that are closer if it's all exponential downward curvature of Earth?
And how did those boats appear to compress together along with the horizon when the camera was closer to the ground? Did the curve do that too?
No, perspective did that. As objects approach the vanishing point (usually on the horizon), they appear to rise closer to that vanishing point. Again, you really should review the basic rules of perspective because they do apply, even on a round earth.