IMO, by far the best 2 are:
1) If you get really close to the surface of the water, it's not that uncommon to have a beam of light travel a very long distance without being obstructed by the curvature.
2) A garden variety physical gyroscope won't be able to show the 15 degree per hour rotation of the Earth.
and maybe an honorable mention might be:
3) Various light-speed tests have been unable to show any motion of the Earth relative to the aether.
1) This was noted as a problem with Robotham's original Bedford Levels experiment. To avoid this, Alfred Russel Wallace repeated the experiment with a modified method specifically designed to avoid the known problems of atmospheric refraction which can and will interfere with (and hence invalidate) the result. What you are saying in effect is, if you get really close to the surface of water, atmospheric effects distort what you see. That's not telling you anything about curvature.
2) And I can't take a photo of the rings of saturn with my smartphone, I need a better, more precise (and more expensive) instrument. It might be possible to design and build a mechanical gyroscope to detect some rotational motion of the earth, but you're inevitably struggling against friction. And who's going to bother going to that trouble and expense when Foucault's pendulum already showed the rotation and the highly accurate ring laser gyroscope does the same job as a mechanical one, but more reliably and considerably more accurately - which is why modern airliners have replaced their old mechanical ones with RLGs. And we all know (thanks to Bob Knodel) that RLGs tell you we're rotating at 15 deg/hour. I'd also add that the earth could still be round and not rotating. Rotation and flatness/roundness are not the same thing.
3) The various tests you mention lead to two possibilities a) If the aether exists then the earth is stationary (does not orbit the sun) or b) if the earth orbits the sun then the aether does not exist. To distinguish between a) and b) it would be necessary to either demonstrate the existence of the aether via some other means or demonstrate the earth orbits the sun. I'm not aware of any evidence of the existence of the aether. Sure it was assumed to exist for a long time, but never convincingly demonstrated. On the other hand there is plenty of evidence to support the idea of the earth orbiting the sun. Again, the earth orbiting the sun has nothing to do with it's shape. The earth could well be round and everything rotates around it - a belief held for a very long time in fact.
Personally, if I were trying to play devil's advocate, I might go with:
1) It looks flat (difficult to get your head around how big this planet is).
2) Why don't people in Australia fall off (hard to get you head around what up/down and level mean on a globe earth)
3) Water doesn't stick to a ball (gravity can be counter-intuitive)