for fellow members of the forum with the above combination (of course and the 20mm extension ring as well), light from the internal flash does escape into and reflects off the dome port, casting a "ring shadow" on the image.
This only happens at the widest end of the tokina - 10mm. I've attached a sample down-res image straight out from raw.

My diving buddy and I managed to gaffer tap the flash such that it doesnt leak out to reflect off the dome, which is a temporary solution while I was diving at Bali. Until there is a more permanent solution, you might want to do the same as well.