I have lost at least half a dozen dive knifes, some were quite expensive. If I don't or can't break it I usually lose it. The other week I lost a kershaw pocket knife that I'd had for some twenty years. Sometimes when I took a knife out underwater, I might not put it back properly or the suit knife pocket or sheath was to blame, mostly I was just careless. These days I take a cheap stainless steel kitchen knife, drill a hole in the handle and cable tie a dog lead clip to it, I use a short length of garden hose as the sheath, and connect the handle and the base of the sheath together with thin bungy or thick rubber band. It clips onto a "D" ring. Taking it out and putting it back in the sheath you can see what your doing, its also no real loss if I lose it.