I dive with a jeweler's loupe. Small enough to keep in my bc pocket, enough magnification to bring those tiny nudis, crabs, fishies up-close and personal. The one I carry now is a 20x. I have to get awfully close to focus, but it works fine most of the time.
Have used different powers, different descriptions, and different sizes. All magnified the items I viewed and opened up a whole new underwater world for me. (Thank you, Bill!)
I've purchased from ebay, craft store and a local jewelers' supply.
Never dive without one, if I can help it.