>In this case, you may have had one additional complication, >which is the large amount of water in the frame - it's all at >a different focus distance, and in this particular case the >fact that the D800 is really fast reacting might be working >against you. The focus tracking lock recommendation is >appropriate for this reason.
I know a pro who shoots swimming meets. He once told me it was a challenge getting properly exposed shots of the swimmers' face if they were swimming directly towards him. The splashing water in front of the swimmers would draw the focus off.