I'd like a blow horn too. However, I'm not sure how practical that would have been, given everything else he was carrying.

Regarding the mirror, I'm not skilled enough to do SOS. My skill level is at being able to hit the target. My goal would be to shine a constant reflection. In this particular scenario (tired, capsized boat, hanging on for dear life), performing SOS with a mirror would be even farther beyond my capabilities. Note also that the rescuer never said he failed to see an SOS signal. The rescuer ignored the flash entirely.