For less than $50, one of the blades I think might work is the Camillus Arclite. Haven't had personal experience with it yet, but I believe the 420HC blade will withstand some period of immersion without much problem and will hold an edge ok. Probably might need a few swipes with a steel halfway through if you're gutting a 2m shark.
Personal experience: 98 Delica and Endura from Spyderco. Have used it to cut line, bait and gut fish every time I go fishing. Haven't had any problems with it, even without the tuffcloth coating. (I left it at home after summer vacations.) Must remember to wash when you get home from fishing. Don't leave it wet overnight.
Alternatively, buy that piece of Talonite being offered as a "knife blank"
here. Sharpen part of it shaping it a little at the same time, wrap some cord around part of it as a handle. There. You have a blade you need never worry about rusting.
I've found a cheap way of making a sheath too. Not very good but will do in a pinch. Plastic milk cartons are thermoformable. You can heat a couple of pieces and mould it over the blade then glue them together using epoxy resin. This will strengthen the set up. Whole thing should set you back about $50 and a whole lot of elbow grease, but you get a Talonite blade.
Good luck with your search.
[This message has been edited by Steelwolf (edited 04-24-2001).]