I carry a whistle, a safety sausage and if conditions really rough, my air horn on my LP power inflator. A mirror is good, if the sun is orientated in the right direction (you could bounce light with your mask and your mirror, with your eyes closed - tricky at best).
I have had to use the whistle once, with a paniced drifiting diver behind the boat, way past the tag line, in rough seas. My buddy was on the transom of the boat, looking for us. The whistle got his attention fast, swimming the tag line off the boat reel to him, securing him, then me. Nasty day.....consider the whistle at a minimum.