Now I want to look at making blades for diving, anybody had any advice to offer?
Yes, several. As a spearfisherman myself, I have been looking for a propper diving knife for a long time.
Spearfishermen and divers will have different needs, so do not try to keep everyone happy with a single product, because chances are you will please neither. You need two different knives at the very least.
Speafisherman use the knife to finish off fish and cut tangles (mono, nets, cordage in general). Therefore we need robust pointy tips, serrated edge and a cord cutting notch could come handy, but is not mandatory. Some like to have features built into the blade, such as holes or notches to pull on the stuck shafts but I rather not to, since it weakens the blade (I have bent diving knives before killing prey).
Divers use the knife for cutting off tangles but do not kill stuff with it (usually), so round/blunt tips are wellcome (so you don't pierce yourself of your gear with it). Some like screw driver tips, o
Steel: Today H1 wins hands down. I am waiting for my first knife in this steel (Orange Spyderhawk sprint) but everyone says it is the best for arround water use. I know that VG10 develops some spots in salt water. And I have gotten other stainless with rust spots.... so better stick to H1. Some others attach/screw zinc anodes to not-so-stainless blades and seems to work ok... untill the zinc anode wears away.
Handles need to prevent gloved wet hand slippage forward so you can get a sure grip and exert force when stabbing prey or doing general cutting. Molded is great. If detachable, make sure hardware is up to the task. Bright colors make them easier to spot if dropped and fish don't give a damn, so you can go yellow or orange. Lanyard hole is mandatory.
For both situations we preffer flat, non-obstrusive, non-snagging sheaths. Molded ABS, kydex, etc. but need to stay flat. Spearfishers will want to strap it to the calf, arm or wear it on a belt. Divers will have more optiones and you should add provison for clips to attach to the vest or even the hoses (hose clamps).
That's all I can think of now, I hope it helps!