Will -

How'd you like a 16-bit cpu with single cycle hardware multiplier that
runs 1 uS instructions at about  1 mA and can power down to <1uA?  It
will run 250uS instructions at several mA and can switch speeds on the
fly.

If so, take a look at TI's MSP420 family.

I don't know what kind of functionality your diving computer has to have,
but my (air only) diving computer runs about 3 years on a single lithium
1/2AA cell.  It wouldn't run that long powered up all the time, but I'd
guess it can run for a couple of months.  It doesn't seem like tri-mix
calculations should be hugely more complex than air or Nitrox.

Incedentally, if you use an offgassing rechargable, and put it in a
sealed case, isn't the case going to explode when you charge it?

I'm interested in your project both as an engineer and as a diver.  Feel
free to e-mail me off-line if you want to bounce some ideas around, or
just keep me posted.  Good luck.

                                Don