guzy, the 0.4x lens I used is just one piece and tightly seal lens so it hold up and doesn't flood, that lens is getting difficult to find:
pubert, your lens will not work, this is because your lens can be taken apart, and use the bottom part like a macro/diopter. What you should try is this one, which is just one lens with manufacturer sealed to make it only 0.3x.
The 0.3x do not have 67mm version. I don't know if you get a 72mm and step down to 67mm, would you get a chromatic aberration or not.
The 0.3x is 180 degree, factor that once in the water, it becomes 126 degree, it's a good angle without making things become fish eye distortion, but the question is what's the quality.