I think a webbing cutter, aka seat belt cutter would be of more immediate use on the water than a knife.

I do have one of those quick release knives from NRS that fit right on your PFD.
It's pretty good except it rusts. I also cut my finger on the water trying to use the blunt nose to pry something open.
I've cut some line with it. Used it more on land than on the water. It attaches and detaches quickly, but you do have to look at it when putting it back.

I've never had to use it to battle sharks, kill pirates or personal water craft brats....

IMHO the most serious and immediate need for a knife on the water would probably be to cut a fishing line (at one end of that line is a nasty hook that wants to be in your cheek or eye). I think an emergency webbing cutter, aka seat belt cutter would be more useful in an on the water emergency than a knife. It would also allow you to quickly and safely cut any line (rope).

The blade of a seat belt cutter is hooked so it's not likely to cut you by accident, to cut a line you just have to hook it on the line and pull (just like a carabiner except you cut instead of attach). The hooking would also catch the three pronged barbed lure before it did any face carving.

If you've ever tangled a fishing line and felt it moving fast across your face, you'll know exactly what I'm talking about.

The black quick release knife on the front of my pfd however has more than enough machismo to offset my girly kayak booties, skirt, puffy life jacket and Tuley hat.....