Common problem in salt water.
Perhaps this will start some kind of huge debate on the topic (and Lord knows I'll find some metallurgist or chemical engineer to argue with), but what the heck...
My personal experience is that in salt water, finding a blade that doesn't corrode is a significant issue:
420 (common) stainless steel is junk and rusts in the presence of salt water, even after rinsing. It holds a great edge, though, if not exposed to salt water. As such, it's very popular to make knives - even dive knives - out of the stuff. It makes an edge well, keeps an edge, and looks like a great knife. Unfortunately, it oxidizes readily. I suspect that your shears are made of 420 stainless steel.
304/316 stainless has much better corrosion resistance. The two differ slightly in alloy makeup, but they're both excellent in the corrosion resistance area. Unfortunately, they don't hold an edge well at all. They're difficult to sharpen, and once they're sharp, they go dull very quickly. 304/316 stainless, therefore, makes for great hardware on boats, backplates, D-rings and the like... But not very good knives or shears.
Titanium, IMHO, does an enviable job balancing the two... It does not corrode easily, and it does hold an edge well. The cost, however, is often prohibitive, and titanium is very light weight and dull-looking... Often lacking a good "feel" or appearance to the diver.
Ceramic cutting tools are a very interesting relative newcomer to the marketplace. Obviously, they will not corrode at all, and they take an edge very well and stay sharp for a very long time. People brag how they can abuse these knives and have them stay sharp. Unfortunately, ceramic knives are comparably brittle, and can break and chip. Chefs who use ceramic knives often take great care with them to ensure that they do not crack or chip them.
Spyderco has recently decided to use a new alloy in their "saltwater" series of dive knives that claim to be both corrosion resistant and able to hold an edge. I have no personal experience with them.
For my disentanglements, I am using an all-aluminum "lockblade" style boxcutter style knife with a titanium-coated 420 stainless steel "hook" blade. The tool sits in my tool basket that I dive with and when I need it, I pull it out and unfold the blade and use it. When the blade begins to show rusting (usually within days of use), I change it out for a fresh blade, which is inexpensive... And I've always got a rediculously sharp "hook" blade for cutting.
On my dive rig I carry a 304 stainless steel Scubapro K3 knife with a much less sharp blade, to use in case of emergency, and to pry and poke with. It's constantly in need of sharpening, which I have done many times. I look forward to replacing this with a Scubapro K3 in titanium, which should provide the same corrosion resistance with a better ability to hold an edge... But look less cool.
I hate the lightweight feel in a knife, too, which is what this will have. Ah... You can't have it all.
Blunt tips, FWIW, for prying.
These shears? Be happy that they're giving you a refund. They're junk.
I don't know if they make them or not, but if you really want the problem solved, see if you can find them in titanium. If not, 304 or 316 stainless ones should do the trick, but don't expect them to stay very sharp. May not be an issue, though, with shears, since you've got leverage that you don't have in a knife.
Alternatively, you can get another set of cheap 420 stainless shears and WD-40 them every few dives - in addition to rinsing them.
I've heard good things about a product called OSPHO by Skyco (
Amazon.com: Skyco Ospho Metal Primer Qt: Sports & Outdoors) and I even purchased a bottle of it. Supposedly, it uses phosphoric acid to etch metal, which somehow prevents rust. I haven't personally tried it, but I intend to. Anything that can help...
Maybe 420 stainless dipped in this might be the way to go? Maybe that's what Spyderco is doing with their "saltwater" series?