It was pointed out a simple problem with flat earth theories and that is gravity.
Water is a free flowing liquid. It always finds the lowest point to fill up from and balance out.
So if we were on a disk, it would have to have a thickness. And that thickness would create irregularities and pulls of gravity to attract the water towards the centre of mass.
If you look at the sun it is a blob of a flowing plasma which balances out into a ball, which makes it almost a 100% sphere with projections linked to explosions and magnetic pulses in the sun. Looking at all the other planets in the solar system the same rules apply. Spheres which pull things together equally. But the earth is the exception, but still has gravity. Now if the disc is stationary, no angular momentum the water would collect mainly in the middle without spreading out. And eruptions from volcanoes with flowing magma would equally flow to the centre of mass so over time a sphere would form and solidify. Gravity would then pull on the extremities and pull them in to the sphere as it grew.
If the earth disk was spinning, the water would reflect the forces. If you sit on a playground round about you get flung off is you do not hold on tight. So the water would be flung into space, unless another force could keep it in check.
A much simpler model is we live on a sphere where the gravitational affects are similar around the sphere. It self balances and matches what we see at all levels.