Here is an example of my bad handy work (top is unedited)
Pitch is the most difficult dimension to stablise with a "still" camera shooting video underwater. Camcorders and their longer housings are much better for pitch stability. There's no easy answer.
Maybe add another handle at the rear of your setup - right hand on housing handle, left hand on the rear handle.
Maybe add a more weight at the bottom and add buoyancy to the top - same weight, more mass.
Wrist strengthening exercises?