Boil it down into pros and cons and see what matters to you most
Cost:
DSLR: probably $650 minimum for body and a reasonable starter lens. Add minimum of $1100 for a housing and port.
PnS: good one for $350-400, another $400 for a fairly high end (e.g. Ike) PnS housing.
Delay:
PnS will have higher shutter delay than DSLR almost regardless. Not to say some of the best higher end PnS's aren't fairly brief on the delay though.
Framing: with a very few exceptions, DSLR's will make you use the optical viewfinder to frame your shots. PnS's let you take the picture with the 'screen' as your viewfinder. If this is important to you, it either limits your selection of DSLRs (and some of those that do live-viewing suffer some shutter delay, probably not the same as for PnS, but some) or points you back toward PnS.
Weight/packing: DSLR is way heavier. My entry-level system (e330, 2 DS50 strobes, Ike housing) completely fills a LowePro Vertex 300 large carryon camera backpack. PnS in housing is much smaller and lighter. Underwater the DSLR is bulkier as well, but possibly more neutral weight-wise than the PnS with the right arm setup. I'd say underwater they can be a wash. I'd even argue there's no additional mental task loading with a DSLR, since a good PnS will want to be shot in at least Aperture priority with manual White balance, if not full manual, same as a DSLR.
Lighting: for either cameras external strobes are the "best" bet. But a PnS housing generally can use the internal strobe, while that's not true for many (but not all, the OEM Olympus e410 housing comes specifically to mind) DSLR housings. So if you do add external strobes, for many DSLR's you need some sort of iTTL (electronic) sync cabling vs a fiberoptic or plain old sensor type slave firing.
Picture quality: here a DSLR can 'win' assuming (as ce4 has rightly pointed out) the right glass for the right job. The higher end PnS cameras have the same # of megapixels as the 'entry level' or enthusiast DSLR's. Certainly not the same as the pros. But megapix count isn't the end-all, as the color bit level, dynamic range, amount of light reaching the sensor (e.g. lens quality) etc all play a big factor. Short answer is a DSLR won't magically buy you a "bigger" picture, but handled right should be qualitatively a 'better' picture in some circumstances by allowing things like depth of field selection and the like that PnS's can't do (even with add-on external wide-angle lenses, can only approximate).
Theft risk, etc etc. I'd say make a list of why you want to shoot underwater, what you want to shoot (e.g. little macro critters, wide angle, etc) and then rank things in order of interest to you. For me, I wanted PnS convenience but DSLR quality, so I went with a (now already outdated) Olympus e330 that I just got the housing kit for and will dive with for the first time this coming week. So frankly, I haven't even TESTED my decision yet to see if it was the right one. 