As your Qs already indicate, all the factors to consider in a knife are multiplied for use in saltwater. Your answers will depend on what you might realistically expect to do w/it. If you expect to pry, you can buy a dozen 8 prybars for $3-$5 each, then not worry if they rust.
I havent been diving in a few yrs but expect to pick it up again starting next summer, so Ive been asking the same Qs as you. My own pref is for an all-purpose knife which I can use in or out of the water; if youre looking at the SEAL Pup, your interest sounds similar. The SOGs 440A is about as corrosion-resistant as steel gets w/the usual compromise, i.e., OK edge-holding and toughness. Ti will insure zero corrosion w/near-unbreakable toughness, but possibly only equal or even lesser cutting performance (tho some forum members have spoken very highly of the Mission ti knives). Thats pretty expensive for a knife which, out of the water, will be outperformed by knives 1/7 its price.
The David Boye cobalt dive knife (4 drop pt, 7.5 OAL, skeletonized handle) can be found for under $100 (retails for a bit more). The Boye cobalt (cast stellite) is completely non-corrosive and is reputed to be an extraordinary cutter which will keep an edge till hell freezes over. Probably a great deal, tho some might find it too small. If you want a thicker handle, you can wrap it w/paracord or rubber.
Again, depends on your interest and budget. Some might say, go w/a cheap knife in the water, but remember you get what you pay for. Of course, on the other hand, if you can afford a ti knife, then why not consider a custom w/ stellite or talonite? BIG price tag and equally big wait, but you get to specify the knifes materials/ dimensions, and youll end up w/a knife which is truly tops in performance, non-corrosion, and utility -- you can use it for just about anything, anywhere. Least thats my reasoning -- I ordered a 4.5 talonite drop pt, which I anticipate (hope?) will be my LAST general purpose knife (do I hear someone laughing?).
My 2 cents --