I would have to see this work to believe it is of any reliability. There are way too many things to interrupt a light signal under water.
Learning sign language is a lot cheaper and it doesn’t need batteries. I’m actually surprised divers don’t learn more than the handful of scuba specific signs (danger; low air; lion fish; shark; etc). My wife and I both sign, so we can have much better communications down under. Learning signs are as easy as watching YouTube videos - or better yet, join your local sign language class (which is how I got started).