Glass magnifying lenses seem to work better underwater than plastic ones, some of which don't work underwater at all. Presumably it's the difference in the index of refraction, or something else involving underwater optics that I don't understand. When you find a lens that works, you can attach your own retractable lanyard, or better just attach a bolt clip.
I have one that I adapted from a lens used to view slides (remember those?) on a light box. It works. And almost every photographer I know has a couple of slide magnifying lenses lying around, not being used.