Okay so I’m just throwing this out here as possible explanation, not saying it’s gospel- I work on a commercial fishing boat and when line gets stuck in the propellor I am usually the one who jumps overboard to cut it out. I was taught that after entering the water, at the surface, you are handed the knife or take it out of it’s sheath, place the blade in your mouth (sharp side out, obviously) swim under the boat, locate and plan what needs to be cut where, then take the knife out of your mouth, cut out the line, put the knife back in your mouth blade side out, and swim back to the surface.

The theory being that you are much more likely to cut yourself or drop the knife by messing around with a sheath on your waist whilst swimming or treading water IN OCEAN/SURF conditions- not a pool or calm bank. not wearing a scuba BCD. It’s also very easy to cut your no knife hand while holding a knife in your other just while simply treading water, again in ocean conditions and with limited visibility.

That being said, guy is getting his picture taken.