I Echo what ssra had to say.
I also have a few SLR's, and DSLR's. But I discovered that putting an SLR in a housing was going to run me at LEAST $1500. For my D1x, more like $5000.
I purchased a Canon S70, and the UW housing for about $550. It does a good job for what it is even if it lacks a few things that really would make my UW photography better.
I have printed 8x10's and they look good. I need to pickup some paper to do some 11x14's, but I'm betting they will turn out well. I don't think I would want to go much bigger, but then again, I'm a bit picky.
I also suggest digital as the learning curve is much quicker. I shoot RAW, and while I try to get stuff right in camera, with the unpredictable nature of UW shooting (especially without a good strobe setup) I find that it is MUCH easier to adjust WB and color balance using RAW images.
I would not purchase a PnS that did not have RAW capabilites, however many do shoot jpg. I shot some jpg on my last trip, but mostly RAW. I can get the images processed in RAW in generally a minute or two. The jpg images I had to adjust were more like 10-20 minute jobs in PS. No more jpg for me!