My two cent’s worth, as a former nautical klutz … all of the safety harnesses I have ever seen/used on sailboats are webbing, generally nylon at least ¾-inch wide or wider and fairly thick. While I always wore a knife on the boat, it was almost always a short, stubby “bosun’s knife” that had a fairly short but thick straight-edged blade on one end and a spike on the other for splicing, opening sheaves and, um … popping beer cans. If you want to get your husband a do-it-all tool he’s more likely to carry, opt for a fixed-blade knife. On the other hand, cutting nylon webbing in a hurry is rough unless you’ve got that I’m-going-to-die adrenaline surge going.
If you’re really concerned about his safety, look into the self-inflating PDFs that are worn around the waist and inflate when a sensor is immersed in water for more than a few seconds. He can slip the knife sheath on the PDF belt and be good to go no matter what happens. It’s not as good as having a PDF on your chest, but it’s flotation if you get knocked overboard unexpectedly (been there, done that.)