Sorry chiming in a little late...
verybaddiver - you WEREN'T being sarcastic when you made your original post?!? Good grief... glad that was put to bed.
If you drop knife, your backup is on your partner. I like carrying EMT shears in my thigh pocket, obviously not as accessible as my knife at my crotch strap.
My Omersub flat knive (similar to the scubapro) came with a soft webbing type sheath with a flap. The knife slips into the sheath and the flap covers the flat handle, secures with velcro (don't like just the velcro, but the sheath is a good fit to the blade). Webbing belt loop on the back barely allows me to slip my SS buckle through. This type of sheath seems easy enough to assemble with some leftover webbing and some creative sewing...
Using bungee to secure a plastic sheath seems good idea, but how would you secure the bungee to the sheath? I'm thinking along the lines of a short bungee tied to form a ring, figure "8" around the top of the sheath and bottom w/crossing point creating the belt loop against the diver's waist. Since the tip of the sheath is tapered, it would be easy for the bottom loop to slip out when you "giant stride" off a boat, for instance. If you make the bungee too tight, it would difficult to remove from the belt... but I guess that would be the only option. I'm not talking about 6-10" bladed knife with a full handle... that would be a pain (literally) up front near the crotch strap.