Re: Underwater Self Defense
Wow! That is such a cool idea -- and what applies underwater would apply in space combat, as well!
I used to be a certified Red Cross lifeguard, and I'd have to say that most of Aikido is very gravity-dependant, and standard techniques would not be effective in a gravity-neutral environment. The laws of motion apply differently -- every action causes an equal and opposite reaction, without gravity for a mitigating force. Grappling is the only way to ensure you get to a safe position relative to uke and can stay there.
I think Aikido principals could be used, but the end results would look more like Jujitsu than Aikido, due to the frame of reference; grappling would be required, because you need something to push against to apply pressure to change someone's posture and intent... simply off-balancing wouldn't have as much "oomf" as it does on land.
For example, jujinage would essentially pinwheel an attacker, but not fling him away, and the lock would fail. Ikkyo would be a kokyunage throw; nikyo would work as a temporary lock (especially the Z-form), but the submission position is really just setting nage up for a nasty strike from "underneath" (the uke's free hand is not required to support the body, so it is useful). Iriminage becomes a good way of removing or threatening uke's breathing apparatus, but unless it ends in a choke or air-source-removal, it's ineffective. Nikyo, Sankyo and Yonkyo would be effective for changing your position relative to uke, but not necessarily for pinning or submission. Atemi would be effective, but be a little too forceful and you push yourself away from the attacker... and back to a neutral position. Again, grappling is the only way to ensure you get to a safe position relative to uke and can stay there.