Scenario Editing Damaging Terrain

So I noticed in the Erik the Red campaign ships take damage in certain waters. How can I do that in the Scenario Editor?
Preferably I’d like to make it so the terrain type deal damage when walking/floating on it, but if the triggers just make a general area deal damage that could work as well.

ZeroEmpires said he did so using triggers at the end of this video, but never showed how.