Huh. At first I wondered how this thread got past me, then I noticed it was ressurected from the dead. *g*.
I'm a little disappointed with SeaSigns - I did try to contact them about SeaSigns as I am a fluent ASL signer and was interested in the possibility of learning the system and becoming an instructor, but I received no response. Either time. Oh well.
A few thoughts about ASL -
As far as most signs being two-handed, it depends on how you look at it. For the majority of two-handed signs, you'll find that both hands are doing the same thing (a symmetry condition), which can be understood even when only one hand is used, or that there's a dominant hand and a weak hand (Battison's dominance condition) in which most signs if the weak hand is dropped can still be understood. It's not necessarily that limiting of a factor, in my opinion. Context is also an important factor, and when you're underwater there's somewhat of a limited expectation of what to talk about..
There was a good point brought up about global understanding of sign language; there is something out there, a lowest common denominator form of gestures, called "gestuno" to my understanding. Unfortunately, knowledge of this is extremely limited within the United States, and does not have quite the widespread exposure.
Be careful with websites such as lifeprint.com, as signs can be misconstrued from static pictures. The best thing to do, apart from actually learning ASL in a real-life situation, is to find a website that shows the ASL signs as video clips. A quick google search turns up the following, although I'm sure there's better:
ASL Browser (Another caveat to keep in mind is that the literal meaning of a word can be signed in several different ways depending on context).
I would strongly encourage people not to refer to signed english sources to learn signs; this is just manually encoded english and often a poor substitute for expressing oneself in an universally understandable way.
I wish there were an easier way to share ASL with the underwater community at large that didn't involve a commercial venture. I'd love to have more folks to sign with at depth!
My best advice is to learn the manual alphabet (fingerspelling) - from there you can build on everything else and have the foundation to learn from other deaf divers (or deaf people in general).
The sign for "cool" (as in "that's so cool, man") is a hand with fingers and thumb outstretched ("5" handshape), thumb pressed against chest, and wiggle the fingers. The sign for "wow" is simply shaking your hand up and down in the "5" handshape, often accompanied with wide eyes.
Works for me....
followed by my buddy's 
