You have obviously never actually tried to set up a Foucault pendulum. It is not so easy. You can't just hang a weight from a rope tied to a tree. That won't work. Also, Foucault's pendulum does not show that the earth is round, it merely shows that the earth is rotating. Flat disks can rotate.

Also, ships do not always sink below the horizon. The right atmospheric conditions can refract the light in a way that causes them to be visible at a much greater distance than the straight geometry would allow. But this explanation can be inverted to claim that it is the vanishing that is due to atmospheric effects and that the non-vanishing ship is the accurate reflection of the geometry. Falsifying that hypothesis is, again, not so easy.