440C with a mirror finish is hard to beat for a dive knife. Divers are compulsive about washing their gear in fresh water when the dive is over. It's the first thing you learn.
Why not something exotic? Because it's not necessary. Keep in mind what a dive knife is really all about. A dive knife is the very definition of a sharpened pry bar. It's a beater that can cut things if need be. Cutting things is the least of what you use it for. 90% of the time you are prying, digging, scraping, or banging on something. You just don't do a lot of cutting while diving. Sure, you want that ability. God forbid you get tangled in something, but unless you are wreck diving or purposely going around netting or rigging gear, divers avoid snags like the plague.
I've had a lot of dive knives over the years. Some good, and some junk. But there are some things to consider in your design.
Good:
-Large, grippy handle. Dexterity underwater sucks. Especially if you are wearing gloves and even more especially in cold water.
-Somewhat rectangular handle profile so that you can index the edge without looking at it.
-Exposed tang. You beat on things a lot with your knife.... because it's the only thing you have to beat on something with. A little bit of tang extending out of your grip is nice to have.
-Square tip. You use your knife to pry quite a bit. Whether oysters, barnacles, rocks, whatever.
Bad:
-Sharp point. Go into a dark closet with gloves on and imagine retrieving a knife. All of a sudden you are very scared of anything with a sharp point.
-Small handle.
-anything that snags. You'd think a lanyard would be a great idea. In reality, lanyards tend to wave around like long, wet hair. They wrap around everything and knot themselves. Maybe make a lanyard hole, but leave it up to the user to actually affix a lanyard if they want to.
Again, just my opinions. Obviously it's your design. This is just based on my experience over the years. The vast majority of my diving is ocean diving, with foot entry through heavy surf and surge. If your nephew is a working diver, you really need to ask him what he wants out of the knife. He may have some specific needs he wants addressed.