Okay, now I'm not a scientist, nor do I claim to be one, but I'm assuming that on the flat earth, the ocean is perfectly flat (meaning if all waves just stopped and there was no movement to cause waves, it would be the most level surface possible). On round earth, if we were to blow up the moon and stop anything from making any waves gravity would make it a perfect sphere of water (Not accounting for the land).
If a ship were to sail across the ocean to another landmass, you should be able to see the ship the other side with good enough optics and a high enough vantage point.
On the contrary, if what I stated above on round earth happened, there would be a defining point at which the ship would appear that it was sinking. No matter how high you went, the ship would still begin to vanish below the ocean at some point.