To do this, I have added a small rock under the water in the middle of the geyser, have added a trigger to it, expanded the trigger to the area I want around the geyser, and added a script to cause the damage.
All works fine in game, and the geyser behaves as expected, causing the damage.
When I go back into the CK in order to start adding this to all the geysers, on opening the CK it says "Duplicate Form ID (01000D62) encountered in file 'Hot Geysers.esp'", "FORMS: Assigning duplicate ID form new ID (01000D65)", and "FORMS: Duplicate form ID (01000D65) encountered in file 'Hot Geysers.esp'", and "FORMS: Assigning duplicate ID form new ID (01000D66)".
What exactly does this mean? Is this normal, or it something I need to worry about? Have I done something wrong in my setting up of the trigger?
I don't want to continue adding triggers to the rest of the Geysers until I know that it's safe to do so!