Nice idea but difficult to achieve universal standardisation and recognition. Look at how
few universally accepted signals there are in o/c diving despite a much longer history and higher user base, even some of them aren't well agreed!
An alternative is to stick with already standard systems:
1. Write it on a slate - assumes you have one to hand and the pencil hasn't broken again.
2. A sign-language spelling alphabet. The two-handed ones would be best suited for our purpose as they are clearer and avoid ambiguity with existing diving signals. This seems to limit it (for english) to BSL (British Sign Language). It is easy to learn and I have used it a few times with my buddy, the only thing it lacks is a "next word" sign.
alphabet.gif