Hi James,
Looks like you have narrowed it down to some excellent choices.
Given that you are planning on shooting divers, a wide-angle (WA) lens is going to be critical. The lenses on the camera just aren't really wide enough (even the S70). Currently only the F810 and S&S have the option of an external lens. Advice from Inon is that the UWL100 28AD (designed for the S70) will not be released until May 2005. I'm not sure whether that will sway your decision. There is a thread here on wetpixel where someone describes using the standard UWL105AD on the S70, but they were facing issues of internal light reflections - so I'm not sure that is a great solution.
Now regarding the F810/SS... I've just purchased the F810 for friend - mainly for its RAW capability and low-light focus speed (which is something the S70 struggles with according to dpreview). I realise the battery is a bit of an issue and I've heard from users that they can only just fill one 512MB card with RAW or JPG before the battery dies (and that is not using flash for every shot). The person I bought the camera for will not be a very prolific shooter, but it may be worth it for you to have the extra capacity. Another thing to note is the F810 does not do STTL with the Inon D2000 (unlike the Canon S70). It will work well in "auto" or manual modes. The auto mode is independent of the camera and relies on light reflecting back at the strobe. For WA work tho, you're probably better off learning manual anyway. Although on the other hand - Alex Mustard had some great WA shots recently from a Oly 5060 and an Inon D2000 on STTL.
As far as the SS camera goes - I've heard very little about it and can't tell you whether it will have better battery life or low-light focus. Someone else might have to chip in there. FWIW it does seem like a relatively expensive option, but then if it performs better than the others - it ought to be. HTH