Dee once bubbled...
There are so many variables...what you expect, budget, expected use, topside or U/w, etc. there's no way anyone can answer your question. The film vs digital subject alone will cause another war of it's own. Film and digital both have their own set of problems and only you can decided what's for you.
Good points.
Here's some other basics:
Strobes:
Once you're below ~40fsw, the absorbed reds are going to cause a strong blue shift, and the only way to solve this is with a strobe. The biggest problem that many of the inexpensive UW cameras (film or digital) have is that they have this tiny little flash that hardly does any good. Its probably a fairly safe rule of thumb to say that your strobe's reflector dish should be at least the same diameter as your lens. The real take-away here is that cost of a good strobe doesn't really vary between systems: this bill will eventually come due, no matter if you're using digital or film.
Digital-vs-Film-vs-Technology:
Currently, this is a never-ending debate. There's advantages and disadvantages to each. But what is definitely clear is the issue of rapid change on the Digital side that affects your UW housing options and choices, as well as longer term logistical support. If you like the camera you have now and choose to put an expensive housing around it, its probably very worthwhile to buy a second copy of your digital camera soon, before it disappears from the marketplace.
Similarly, there's a lot of technology items that are more important than others for UW photography. Macro is one. Another is missed shots due to shutter lag, and for this you have to go pretty high end in digital to get a shutter release that's "immediate" like you get on a film SLR.
Logistics:
A longer trip where you take a lot of photos is going to require some additional logistics in order to store them until you get home. This is one area where film still has an advantage. Digital has caught up with the invention of "digital wallets", although this is yet another $250 investment to make in equipment.
Two additional points:
On logistics, a couple of years ago, I was on a non-diving weight-limited trip (35lbs for everything) and digital wallets had yet to hit the marketplace. I was thinking about borrowing a nice digital camera we had at work, but when looking at the "many shots" question, calculated that I'd have to carry $2000 worth of CompactFlash cards, if I assumed that I'd be deleting 50% of the images as I went along. Today, the price for digital storage cards have come down, but it still can represent a very significant investment, particularly as MP's keep going up. And even though these cards are very small, digital cameras suck batteries, and because of this, there was no "full package" weight/volume advantage to going digital (vs the bulk of 50 rolls of 35mm film & 2 backup lithium batteries for the SLR).
If I were to lose my UW film camera tomorrow, I think I'd be really tempted to go to a digital video UW system (especially if you have a Macintosh and iMovie software). And since many of these can also take stills in the ~MP range that are good enough for the web, its something else to consider...
-hh