I have read most of the post of this test and I see quite a few suggestions for folders. I guess all come from well intended suggestions but none or little experience while diving/freediving (I don't want to offend anyone).
Solving an emergency underwater (while freediving with no tanks) is pretty similar to a self defense situation. You will rely on gross motor skills and fumbling arround with a folder is asking for trouble. In my opininon, it is mandatory to be a fixed blade to avoid any extra steps (such as opening the folder).
Also, the sheath must allow to be secured in a readily accesible and non obstrusive area of your body. I, being a rigthy, carry it strapped to the inner side (between the legs) of my left calf. This allows reaching and grabbing it with my right hand easily (not so easily with the left, but doable) while at the same time avoiding tangles with the rope that I use to drag the float (mandatory by law, to avoid being run over by boats).
Some of my friends carry it on the arm, handle down (upper arm near the shoulder or forearm), but I feel that this location will make it totally unnacesible with that arm.
Calf carry allows for a bigger blade. arm, chest or belt... call for a shorter item, as it gets uncofortable quickly.
Regarding the sheath, I made a kydex one for mine (same I intend to do with the Spyderco Fishunter) so it lays flat against my body. It locks securely but a firm push lets you draw the knife. Also, there is a bungee cord lanyard loop from the lanyard hole of the knife that I tension over the lower tip of the sheath, so it makes positive tension on the knife against the seath. I haven't lost a knife since I use this setup.
The scuba tech divers are a whole different animal. I belive they carry several cutting implements. From chopped down (2"blade) Victorinox steak knives with a molded pvc sheath, to utility blade strap cutters, to trauma shears, folders... you name it. Freediving spearfishermen... don't. Fixed blade only.
Corrosion resistance is only one of the concerns for a diving knife.
Mikel