Stephen Frink has posted a few underwater images taken with the Canon 1D Mark IV, as well as his first impressions:
Holy crap … AF is so nice. Very fast and accurate, even in low contrast subjects like black wetsuits. Focus acquisition is brilliant. Having shot my 5DII in similar conditions the 2 previous days, the improvement is striking.
Check it out, if you’re thinking of taking your 1D Mark IV underwater.