If you do I recommend something with a serrated edge. Back in my Army days we were disposing of some old seat-belts so we tried out a seat-belt cutting tool that one of the other crewchief's had that kind of looks like one of those big enclosed envelope openers. It didn’t work as well as we thought. You had to keep backing it off to get another slice because the belts kept getting bound up in the guard. We were thinking that was probably something you wouldn’t want to have to worry about if your car was sinking or you were already underwater. A blade with a serrated edge went through those belts like butter though.
I also had to train in the “dunker” (a cage that is supposed to simulate a helicopter landing in water). The instructor knew we carried knives and selt-belt cutters so he reminded us to only use them as a last resort if the seat-belt wouldn’t release. I guess people get in the mindset that they have to use their tools even if the usual method would still work and is much quicker.