Not very recently, but for decades on a pretty regular basis, maybe half the times that I've needed to use a knife aboard a sailboat. One hand is keeping you attached to the boat- rigging, railings, whatever, and that's keeping you out of the water, or worse, aloft. That hand is keeping you alive. In weather, no, you do not even have that one second to use both hands, you HAVE to hang on. It's a world of rope (lines) and fabric (sails) and if things go seriously wrong you may need to be able to cut with some urgency.
A sheath knife works, but pretty much no one wears them anymore, at best you get way too many raised eyebrows around the marinas. Back in the '60s and '70s there was a fair amount of demand in the small boat world for.. well, automatics, though they weren't called that. Those were pretty much the only one-hand folders then. The quality was generally lousy, and they were considered highly illegal and almost impossible to get, but sheath knives were more accepted back then, in the days before Mrs. Grundy ruled absolutely everyone and everything with an iron fist.
On pretty much any sailboat with a crew the rule was that no knife went aloft without a lanyard, one attached to your body, not just a rat-tail. There were people working below you. Even when you're alone, in that world anything you lose hold of, anything you drop, is usually gone forever. I really hate lanyards on knives, but aboard it's the lesser evil.
Knives are a constant need for doing serious work on the water. Fishing nets are a threat not only to those on fishing boats where the damn thing may be trying to pull you overboard and then pull you under, but to divers. Getting seriously fouled in a near-invisible net underwater without a knife is not a good way to go, you may be struggling there until your air runs out. In any of those scenarios you may have only one hand for the knife because the other is what's caught.
I'm surprised no one else has mentioned it.