As apparently confirmed by the rescued diver's report, a low-powered laser seemingly offers some real advantages over other common active and passive signalling devices, under some circumstances. At night, or when atmospheric conditions render the beam visible off-axis, it must surely outperfom any other visual signalling device by a wide margin at distance. Of course you don't want to shine it directly in someone's face, but that concern seems wildly overblown. Maybe carry the laser for distance, and more typical light or strobe for close-in.