If you go steel Marine Tuff Cloth makes a huge difference in corrosion resistance.
Also, thoroughly rinsing drying the knife & sheath asap after the dive. Leaving a steel knife in its sheath un-unrised is basically a rust incubator.
I would really assess what kind of diving you do and how frequently you do it to determine what kind of knife to get.
Sight-seeing, spearfishing, tech-diving? Twice a year or twice a day?
If you really dive all the time or just plain dig the cool factor then maybe it's worth investing in a Ti knife but it's not absolutely necessary.
I've carried all sorts of knives when diving and for the vast majority of my u/w cutting tasks I would have been best served by a pair of EMT Shears or a dedicated line-cutter.
http://www.omsdive.com/cuttool.html
That's because the vast majority of my cutting tasks have involved cutting line of some sort. Not puncturing things or prying things. I want to pop the line quick and move on. Line cutters rock that way.
The tiny remainder of my cutting does involve puncturing...Fish. A pointed knife when spearfishing does come in handy but less than I would have guessed over the years.
IMO folders are super handy dive knives. I always have an EKI CQC-7 which covers the puncturing tasks, is easy to carry and is surprisingly corrosion resistant.
I can carry the line cutter and the folder on my BCD instead of my calf which is way more convenient to access. This carry position becomes more than simply convenient if diving in enclosed spaces, caves/wrecks.
As for all the prying style dive knives? I actually have never had to pry but if I thought I was going to do some prying (light salvage) then I would bring a pry bar instead of a knife pretending to be a pry bar.