What's most important is that the hand signals are standardized, clear, and simple...especially the important ones: hold, dive over, and OOA. No other signal should be similar enough to cause confusion with the other ones.
Command signals such as "hold" and "thumbs up" need to be responded to by repeating the signal back to your buddy, otherwise I have no idea if you understood what I was saying. New buddy teams should go over and agree on signals before getting in the water, especially the command signals.
Signals also need to very clear, not sloppy, casual, or rushed.