I'm Using the Kenko Pro 300 DX.
seems fine to me, 2 stop light loss as normal. but my images are sharp
i take it your not actually going to use a 70-200 underwater ?
here's an image shot with a sigma 70-210 F2.8 with the kenko attached.
Click to view attachmentthis was shot using a monopod from approx 6m
regards
craig