A knife for Kayaking and a "Dive Knife" are two different things. A Kayaking knife ideally is a 2-4 inch blade mounted for easy access.
Many mount them on their vests, handle facing down. The size and accessibility is similar to a pilot's survival knife because you are basically sitting in a cockpit. A "Diving knife" depending on the type of diving and where, can range from 4 inch
blades to 7 or so. These larger knives are used in scuba in low visibilty, deep, and wreck dives. Clear shallow warm water diving doesn't call for such knives. Don't let the steel type influence you too much; unless you work underwater in the ocean, or absolutely refuse to ever wash down your gear, you won't have a rust problem. The only exception is any carbon steel. I have a dive knife in Aus6a that
I bought in 1973 as a kid and it's still in fine shape. Still, if you are into the latest rust free steels, look at the G.Sakai Sabi models in H1. The Sabi series are fixed, come in various sizes in kydex sheaths are specifically made for saltwater applications. This is the compay that makes Spyderco's H1 knives. If you are determined to wear a knife on your lower leg (thigh) which is where I use to wear my dive knife, the cheapest solution is a SOG seal pup in kydex sheath with rubber straps. Any of the larger dedicatd Diving Knives will work this way.
But as already pointed out, any thigh worn knife may get in he way if using a sit-in Kayak.