When scuba diving, it is important to be able to cut your way out of tangles. This is especially true if you are diving in places where people go fishing. It is difficult to see monofilament fishing line underwater. Entanglement is a real hazard.
For this reason, most scuba divers carry a dive knife with them. It is also a good idea to have an easily accessible backup, just in case your normal knife is lost or cannot be reached.
I own a Dive Rite Z-Knife line cutter as a backup.
It is based on the design of a parachute line cutter for cutting away a tangled chute before opening the spare. It has a razor sharp blade, and its shape minimizes the risk of accidentally cutting yourself when using it.
I like it, but there is one disappointing drawback. If you happen to try to cut steel fishing cable, you will only succeed in destroying that sharp little blade. It also has the tendency to rust over time. (Mine lasted a few months before rusting, but only one trip before I tried cutting something it couldn't.)
They don't sell replacement blades.
Or at least not ones that I could find.
These people couldn't find them either.
These people had some ideas, but nothing readily available.
So. Since it appeared that the only option was replacing the whole thing, I had nothing to lose by trying to modify mine.
First, I removed the old blade. It measured 26 mm by 8 mm.
Next, I scoured the internet trying to find an identical replacement. No luck.
Finally I went to the local hardware store and found this. It has a stainless blade (not sure what grade, but we'll see how it lasts), and the blade width of 9 mm is very close to the 8 mm blade I removed.
I broke off a section that consisted of four segments of the blade, and ended up with this piece. It is remarkably close in size to the blade I removed.
Next, I partially pushed in the new blade. It was a little difficult. I could tell that the blade was larger. To fix this, I used the pencil torch from my arsenal. I heated the point of the blade for a few seconds and then used needle-nosed pliers to pull the hot blade into place. This melted away some of the plastic, making room for the different-shaped blade end. It still interfered with the screw hole after the first time, so I pulled it back, reheated, and pushed it a little further. Below you can see the before and after.
Using the needle-nose pliers, it was pretty easy to remove and install the blade at this point. (I took the comparison pictures of the two blades after doing this. That's why the tip of the new blade is discolored in the pictures at the beginning.)
Here you can see the final result. The blade is firmly held in place by the screw.
There was enough left of the box cutter blade to make 3 more of these replacements. We'll see how long they last, but this appears to be much more economical than replacing it each time the blade is damaged or rusts.