Don't try to apply logic to their argument.

They don't understand it, and it's mean to talk over their head. /s

Also, no matter where you are on Earth, that water mountain is always juuuust over there.

Well, under certain conditions, it does.

https://en.m.wikipedia.org/wiki/Fata_Morgana_(mirage)

From shore it looks like the ship goes up doesn't it? But what is happening when you're on a boat and go on the ocean.? Maybe the water mountains are like silicon breasts as you ride along them they depress under the weight?

Maybe it's that the land is on top of the water mountain/ plateau? And as the ships sail away they descend from the plateau? Maybe it's the gravity of the land that causes the water to bunch up near the land? Maybe the earth is actually spherical? Wait no not that last one...