I was a Merchant Marine officer and went to sea for a living before I retired this year. You wouldn't expect the horizon to be curved even on a spherical earth. As a simple experiment, take a small stick and a string and put it on a globe, or even on something like a basketball. Using the string and the stick, draw a circle on the sphere at a constant distance from a constant height above the surface. You come up with a circle at a constant distance above the known point. This is what you are seeing while standing at the rail of a ship. I have been on a ship where you can see in a 360 degree circle. The limits of your sight are the edge of the spherical earth and, because it's a sphere, the distances are the same in any direction you look while you are at sea. What proves that the earth is a sphere is the ship's radar. I know for a fact that one of our radars has a range of 35 to 40 miles. We know this because while going by Mt. Fuji, in Japan, we can see the image of that mountain and it's at a known location. With that kind of known range you should be able to see another large container ship on the radar at 20 to 25 miles out, but you can't. That's because the ship is below the horizon due to the curvature of the earth. We know that the ship is there because of other tracking equipment we have aboard but we just can't see it until it's about 15 to 18 miles away. The taller the ship, the farther out you can see it both with your own eyes and with the radar. Since Mt Fuji is so tall, you can see it out at sea for quite a distance. As you get further and further away, the mountain appears to sink into the ocean. That kind of visual clue would only occur with a spherical earth. You see the same kind of thing while approaching the coast near a big city. The tall buildings will always been seen first but only the tops. Slowly the city appears to rise out of the sea until as you get really close you can then see the docks where you are going.