Hi @Fajer ,
Nice to hear you have it running with reasonable results!
The easiest way to perform a simulation with a detector in water would be to set world_material = "WATER" in [GeometryBuilderGeant4].
Another way would be to change in the model configuration, and add support material with material = WATER around the sensor, if you want more control of the shape of the water.
An example of this would be to add
[support]
thickness = 20mm
size = 30mm 20mm
location = "sensor"
material = "WATER"
to the model file, which will add 20 mm of water in front of the sensor. More info about placing passive material in this way can be found at Detector Models | Allpix Squared
Kind regards,
Håkan