Well, even if the OP had had a buddy and had gone into the water with him, all that would have meant in the end was two people on the surface, trying to signal the boat, right?
I'm not sure I would have waited 20 minutes at depth, though, although if you could see the dinghy, you at least knew they hadn't returned to the boat for some reason.
Off any boat, I dive with at least two lights, a DiveAlert, and an SMB, and my buddy has the same. I gave up the mirror after reading an article on what was visible and for how far, and it is my understanding that, unless the place where you are diving has the technology to track EPIRBs, they aren't useful -- so I haven't spent the money to own one yet.