CRKT:
MUK- You'll need to get a Kydex made. There is a maker of the forum here with them in stock. If you like the looks of this one drop me a VM I'll find u a link. I'm taking mine with me to the gulf. You just can't beat the price!
It's on it's way to you right now! Enjoy!
To the OP...
1. I suggest a fixed blade knife, so that he doesn't have to worry about opening it, just using it to do the job he needs done.
2. Stainless or an Exotic Alloy like Spyderco puts in their Salt knives would be good, less to maintain.
I have a Spyderco Aqua Salt on loan from a friend, and it is really great. Made with the exotic alloy H1, it will not rust. And it also comes in a high visibility yellow handle, and un-serrated edge, if you prefer.
3. You might also consider a knife without a point, so that he does not stab himself. There are probably many opinions on this, but you will find many dive knives without points, for safety and other uses.
Spyderco makes the Caspian Salt. This is is the blunt one, they also make a pointed one.
And Hultafors makes a knife with a dull point... (top one)
4. And finally, I will justify the type of work I do (making sheaths for knives that already have sheaths)... If you really like a knife, and the sheath that it came with does not suit your purposes, then a custom sheath can really make that knife packable and useful again. And let's face it, the unfortunate truth is that a great many stock sheaths are not super great, adequate maybe, for some tasks, but not always what you are looking for.
For example, I have a knife by one of my favorite manufacturers, and it came with an adequate sheath for most things, but I wanted to carry it inverted (upside down) in a shoulder harness, and the sheath was just not up to that.... So, I made a new sheath, and now I really get to enjoy that knife.
Just my .02 cents....
Best,
Marion