Investigate Inon. They have just what you need.

FYI, Canon makes two different "grades" of waterproof cases for their
P&S cameras...some are good to ~3m and thus only good for snorkeling,
whereas others are good to 30m-40m and thus are suited for scuba.

Unfortunately, it doesn't appear that Canon offers each "grade" for
each camera they sell, so it is possible to buy a P&S and not have a
compatible 40m housing available for it.  The good news for you is
that the WP-DC5 is one of the ones that's good to 40m so you don't
have a problem.

FYI, it appears that Canon might encode this performance parameter
into their product number...the 3m (snorkel) housings seem to usually
be "AW-" numbers (All Weather), whereas the true underwater housings
are "WP-" (Water Proof).

For a full product list see:

<http://www.usa.canon.com/consumer/controller?
act=StandardDisplayAct&keycode=underwater_photo&fcategoryid=113>

or:

<http://tinyurl.com/2ge5ms>

There's a couple of different strobes that could be made to work.  In
general, what most of them will probably have in common is that
they'll be triggered as a Slave off of the existing built-in strobe of
the camera.

The current B&H catalog (I just happened to get one in the mail this
weekend) has one product that has its slave strobe sensor on a wire &
pad and you attach the pad directly to the housing, right in front of
the built-in strobe.  I'll see if I can relocate this product tonight.

The other thing to keep in mind is how to attach it.  I've found that
some UW housings have a standard "tripood mount" female thread on
their bottom, to which one can attatch a (tripod, or) camera tray.  I
recently found that mine (WP-whatever, for a Canon A80) does so, and
I'm able to attach my Ikelite tray for my Nikonos V camera, to which I
have an Ikelite SS-200 strobe (with Slave Sensor for it to be
triggered).

I have done a couple of hours underwater with this setup and have
learned two important things about it.

First, using TTL exposure control is pretty much out.  The camera
doesn't know that the Slave is going to fire, so its pretty easy to
get too much/too little additional strobe lighting.  As such, you're
probably mostly looking at going to manual settings in order to get
good consistency.

Second, because of the in-camera strobe, you're going to get seriously
bad backscatter potential.  As such, an ideal slave strobe setup will
have some sort of physical panel to block off the built-in strobe's
illumination, to minimize the amount of backscatter from suspended
particles that are close to the lens.   If you're not familiar with
backscatter, here's an illustration of the principle:

<http://www.huntzinger.com/photo/2007/backscatter.jpg>

-hh
-hh