I should correct you and note that this is not a "rookie" mistake. I've been shooting underwater since 1968 and still make similar mistakes all the time. And Stoo please note that I would never get to 120' without noticing I left the lens cap on. When it happened to me I noticed it at about 60'. Please note my avatar photo, yes that's a lens cap I'm holding. It's very hard to remove the cap unnoticed on a little panga filled with your U/W photographer friends. Thanks to Allison Vitsky Sallmon for capturing my pain for posterity. I'd like to say Tim that I've never jumped off the back of the boat with no fins, but I can't.
Clearly the answer Mr. Tales is to shoot manual, that way you must remember to actually control the camera. Or said differently you can then screw up the shutter speed and ISO too.