For UW, I shoot slides almost exclusively, but then again, I'm not using
a Sea&Sea MMII-Ex.

If its macro, I'll shoot Fuji Velvia (the "old" ISO 50 stuff only...the
new stuff that's ISO 100 isn't as good).  However, ISO 50 is harder to
work with for non-macro UW stuff.  

For general "clear warm water" work, I'll use Fuji Provia 100F, or the
consumer version (Sensia; both are ISO 100), and for "not as clear"
water, Provia 400F (ISO 400)

Yes, there's the narrow exposure lattitude problem.  I merely live with
it and bracket, as this is easily done on a Nikonos V.  

Agreed, although it also depends on the interest for the primary
application, as digital scanning is pretty cheap these days, and once
its digitized, the cost for an inkjet print is the same regardless of
the original's source.

I'd probably not recommend slides for a Sea&Sea MMII-Ex because I don't
precisely recall how easy/hard it is to bracket with this camera, but I
don't think its particularly easy:  I do know that since it has Auto-DX
coding, you can't fool the camera's TTL system shot-to-shot (by changing
indicated ISO) in order to manipulate strobe output control.  This is
the traditional old Nikonos "trick" to avoid overexposing highly
reflective fish such as Tarpon & barracuda.

Overall, I'd recommend a print film.  I'd suggest:

Kodak Ultra 400UC 135-36 Professional Color Print Film (ISO-400)

Kodak Ultra 100UC 135-36 Professional Color Negative (Print) Film
(ISO-100)

Both are very high saturation professional print films that try to mimic
Fuji Velvia slide film.  Because they're professional films, you won't
find them in Walmart shelf, or even most typical camera stores.  They're
around $5/roll from B&H and other professional service retailers.  Go
online and buy a 5-pack for $25 and give it a shot.  If you like it,
plan for your needs and place another order...FWIW, my orders usually
run somewhere between 30-60 rolls, and I keep it all in the 'fridge at
home so as to prevent any concerns about film expiration dates.  With
refrigeration, don't worry if you go a year or two beyond expiration.

Regardless of slide-vs-print, film is now at a disadvantage versus
digital for delivering prompt feedback on how you're doing.   IMO, its
generally a good idea to get at least one roll of film developed while
still on vacation when its a new, untested camera system and new,
untested UW photographer...far better to get "ripped off" for $40 or
whatever for a single roll developed than to get back home and discover
that you had your snorkel blocking all of your shots on all of your
rolls, or similar correctable problem.

-hh

Only because they weren't willing to wait around for the supernova that was
necessary for fill light for Kodachrome 25... <grin>

Kodachrome 25 had a finer grain that 64... The colors from either were so
much more billiant than what you see these days with the current crop of
films... I mean, really, who is gonna write a song about FujiColor?

http://www.verticalwind.com/k-25.html

I was taught to use transparency (slide) film *becuse* of its narrow latitude.
Its pretty hard to learn about exposure when the film you're using covers up
your mistakes. OTOH, if the camera ihas automatic expsoure control and doesn't
provide a means to tweak the exposure, then you probably have little choice
but to shoot reversal (print) film.

As far as the live-aboard situation, its pretty tough to print from negatives
without taking up a lot more time, space, water, and power aboard the boat.

I found that getting decent (as in 8x10 and larger, hang-on-the-wall quality)
prints from *any* film to be difficult and expensive. Not that I tried all
that hard, but after using a number of local and mail-order labs and burning
over $500 in experiments, I gave up. (That was 3-4 years ago.) I've had much
better luck scanning slides and printing myself on an inkjet printer. YMMV.

Film processing on live-aboards is probably going away. In November on the
Kona Aggressor, they didn't even have a fresh supply of chemicals, or a drum
that could hold more than one roll. The crew, and their cusomtomers, are going
digital. I'm a dinosaur, shooting film, it seems.