To use the PolarPro DiveMaster filters with your GoPro Hero 9, you do need a protective housing. The DiveMaster filters are specifically designed to work with the GoPro Protective Housing, which is compatible with the Hero 9, 10, 11, and 12 models.

The filters will not fit directly onto the camera without this housing, so you must have the appropriate protective housing to use the DiveMaster filters effectively.