As far as ASL goes, the problem I run into is the very same problem I had with the Radio Shack "walkie-talkies" I got for my ninth (or so...th :biggrin

birthday: you need at least two people to learn it. I have a few buddies with whom I dive more often than with others, but I don't dive exclusively with them, and they have little incentive to do homework for what little gain they can perceive. Learning any method of communication (spoken, signed, or signaled) without having someone with whom to practice is not well within my abilities.
Frankly, I'd really like a buddy to learn code. With the low visibility we often suffer (and enjoy :biggrin

, being about to communicate by "do-do-do"-ing a little "Tom's Diner"-esque bit of Morse through your regulator would be entertaining and convenient. (Plus, it needn't carry far. It's not like we'd have to quack it to each other -- heaven forbid!)
What better way to communicate underwater than by dooting it out through your regulator? You can have both hands occupied while facing opposite directions and still communicate with impunity. (No need to lose communication just because you're back to back and being surrounded by a ravenous herd of less-than-happy man-eating sharks, eh? :biggrin
Hehe,
anyway... "normal" signals, ASL, code, slates, etc. can all be used to communicate. Choose from among them as you wish (and as your buddies wish). Personally, the "slate-a-tron" concept (i.e. bunch of words, letters, and numbers pre-written, like a fancy McDonald's register) is something I may have to consider. With a bit more thought to the efficiency of the layout, it can probably be made into a rather useful communications tool -- it can signal things outside your vocabulary, just like finger-spelling, but it requires no prior knowledge on the part of your buddy.