The area I was in is the top of the Chesapeake Bay an area with slow moving tides. If the buoy was moving up or down the boat was moving up or down.
I also went back out today on the boat and was amazed at the difficulty of seeing the buoy during the day. We could not see it even with binoculars until we were about 1 mile away , and it very difficult to see even then. At night time the buoy is very clear. The point here is that you might suspect that curvature is hiding it during the day.
Robby and I aren't necessarily disagreeing with your conclusion, just the calculations you used to reach it. If the Earth is round, then the horizon is what limits your line of sight at a certain distance. As such, the relevant issue is the projected distance by RET of the line between the limit of your line of sight and the object. As per Robby's diagram, this is not a vertical line, but a perpendicular line from the tangent formed by the object.
All Robby is saying is that the vertical drop-off from the observer to the object doesn't matter, because according to RET the object is supposed to be a given distance below
the horizon, not a given distance below the observer. However, despite the mathematics being wrong, your results still contradict RET, as the object should still have been below the horizon if it was caused by the Earth's curvature.