From point and shoot to DSLR (you don't want an SLR as that would use film)?! That's a big leap in cost. You are probably looking at $5K USD at least: $1000 at least for a lower end DSLR, then $1500 for good lense, $1500 - $3000+ on the housing, $750 for a strobe, a few hundred on memory cards, $350 for land flash because many DSLR have no flash. Those are ballpark and don't add up all the little expenses that eats up a few hundred more at least. Pelican case to carry it in for example.
If you want great pics on land and water invest most of the money in lenses to get the best pics. There is a BIG difference between consumer grade lenses and pro lenses and it is worth every cent to get a pro quality lens. For Canon, that would be L series lenses denoted by an L in their name. Nikon has their line. Not only sharpness but things like bokeh which is how the out of focus stuff looks. Yes, that does help a lot. Pics you thought were blah can have a artistic look without you trying. The box you keep the media in i.e the body, does not have to be top end, just get best image size possible and look at frame rate for shooting motion.
Decide what you want to use i.e. pay for, on land then see if you can get a housing for it. Getting one should not be the issue for Canon or Nikon.
Note these cameras also shoot HD video meaning you can drop having separate devices however there are some drawbacks and advantages to using for video.
Then you have to lug all the junk with you when you go diving. The learning curve to get good images underwater is generally mountain steep. I just shoot video as it is easier to much easier and I can show someone what it was really like moving around rather than a cool still image.
If you are trying to improve shutter lag, then you can get better quality point and shoots, but when you step into DSLR you automatically need more sophisticated housings, i.e. bigger money. It's tough to get a setup in the middle of the range between point and shoot and DSLR. It tends to be the bottom cost and quality or high end cost and potential quality. Realize that having the monster rig doesn't mean you just jump in and shoot like Nat Geo. There is still a huge amount of learning about lighting and color underwater. If you are just going to shoot a bunch of blue pics and be done, then I suggest it isn't worth the money. Also remember to enjoy your dive on not spend the whole time trying to document everything you see, as an endless roll of individuals isn't so exciting but wide angle is not easy either. Most of this was learned through painful expensive experience on land and sea.
