Much below 3m and the light drops off a fair bit so the Oly Mju *** SW will be using higher ISOs - NOT where it excels! Also, as someone else has pointed out, 10m isn't that deep when SCUBA diving and that's as deep as the Olys are supposed to go.
Maybe a book on underwater photography is the way to go - it's possible that the walnut could get some good ideas if he does decide to hire an underwater camera. FWIW, a diving friend finds that having a camera with him is a major distraction so maybe not the best idea for a relatively novice diver.