Ok I found out. Hear me out, if you want to create a WaterBodyMesh :
- Create a WaterBodyCustom (if you want to do a Blueprint, just inherit from WaterBodyCustom)
- In the parameters, make sure you have a water material selected like Water_Material_CustomMesh
- You can delete most spline points because it won’t matter, except for the water buoyancy level (just keep one)
- in Rendering, open the advanced parameters at the end of the list and select a mesh, for instance Cube
- Set the water level accordingly (for Cube, you can change the only spline point to z position 50 for it to coincide with the cube top face)
- Make sure to check Auto-activate, so that buoyancy works