But that's what I was saying about about the angles of incidence and reflection. As a simple example, if the boat you want to signal is directly in front of you and the sun is near the horizon 60 degrees to your right, the mirror must be pointed halfway between them, i.e., 30 degrees to the right of the boat, for the sun's rays to be directed at the boat. If the sun is high in the sky the same thing happens in three dimensions. Pointing the mirror directly at the boat won't work unless the sun is low in the sky straight up from it.I have a signaling mirror I have attached to my BCD from DAN I think. In the middle it has a little optical you look through at the target of the flash. Then you wiggle it.
I like the idea of the two fingered "V" way of aiming the mirror (call it the Agincourt Method) that inquis described.