We've all heard stories of divers being lost at sea, the dive boat forgot them, etc.
As part of a small kit of emergency type of kit, here's a rescue signalling mirror that I keep in my BC in it's own plastic ziplock pouch. It has a lanyard on it.
The instructions are shown on the back as below:
Don't forget to attach a brightly colored lanyard maide out of something that floats and is waterproof like polypropylene, althoigh one a little thicker than shown below.
This mirror as well as an awesome set of really good visual instructions can be found here