A water ball spinning at 2000 mph requires much more mental gymnastics than a flat Earth. Talk to some airline pilots and theyll set you straight. They're told on the first page of the pilots training manual that they will be flying over a flat plane.
Flat, calm water would be impossible on a spinning globe. The Earth's atmosphere would be sucked into the vacuum of space if Earth was a globe in space.

My father-in-law is a commercial airline pilot who definitely believes the earth is round.

Water can't fly off into space because Earth's gravity pulls it down in the same way the moon pulls it around to create tides. In addition, Earth has an atmosphere and a strong magnetic field, so the vacuum of space isn't sucking anything anywhere.

The oceans may appear flat, but you can easily notice the curve by watching ships sail into the distance. They disappear from the bottom up because they sail past the 'hump' from your line of sight. Same thing is visible watching cars drive away on the salt flats.

Put a single drop of water on a flat surface and take a look at the top of it. Water can definitely curve.

Think about a flat earth, the distance that pilots or sea captains would need to cover in order to navigate below the equator would be noticeably longer than above the equator—how would they not ALL have gotten suspicious?