So let's assume the earth is flat.

You sail to the edge of the earth and what happens?

Well this ice wall theory is kind of far-fetched. It's more likely that you reach the edge of the earth and something happens. Say you're sailing and you reach the edge of the world. It is very likely that some force keeps people from actually crossing over the world. This force could be anything from illusion to strong winds. Maybe you just cease to exist.