I have a cheap, flat Taiwanese "United"/Stingray (ebay) knife -probably the same as the one you lost -- in a flat Limpet/Pancake sheath on my inner calf. Mine has been deliberately aggressively sharpened over time, so it is somewhat slimmer than normal. But I usually dispatch fish with my fish stringer's spike. I started carry a home-made iki-jimi spike on my belt recently.
I've considered the following knives in the past (also see older threads):
[*]RA Assegai (flat, v. expensive, only half handguard); a couple of other companies (e.g. Pelaj) offer similar products,
[*]
Mares/Sporasub Snake (v. expensive, perfect dagger shape? Zinc anti-corrision butt) [see image near bottom of post].
[*]
Omer Laser USA (inexpensive, no moving parts, simple sheath, same blade as Omer Mini Laser mentioned above & endorsed by StuckInSurrey
, simple but effective rubber ring holder)
[*]Imersion Mini dagger - (inexpensive, slim spearo dagger, 2 variants available).
https://www.apnea.co.uk/product_info.php?products_id=1139&osCsid=c9c24e3c9d2f5801001dbe734450c08e
[*]
Beuchat Mundial dagger - looks cool but a little plasticky.
[*]Old Omer dagger - looks cool but early reports of quality problems like loose handles put me off. (You can glue them yourself - but you shouldn't have to).
[*]Seac-sub Samuri - small, cheap (ebay/Paignton) dispatching knife for arm/belt (similar to Omer Mini Laser but blade shape not as good IMHO):
http://www.scubasworld.com/proddetail.php?prod=seacsamuraiknife&cat=13
[*]Hattori dagger - simple, flat diving dagger (medium price, cheap for a Hattori). Blade seems a bit broad to me but its Japanese
- and the Japanese love good blades.
My current preference is for the
Omer mini laser USA or an iki-spike (for dispatching, fixed on belt or arm) and/or the
Mares/Sporasub Snake (for general/emergency use, fixed on leg/calf) - although I'd miss the sleek unobtrusiveness of the flat RA limpet sheath and security of the unbroken elastic leg bands (although buckles are probably easier to put on).
Still waiting for somebody to come out with a cooler profile flat, slim, affordable spearo dagger - preferably in a flat tape Limpet/Pancake sheath. An Omer laser or mini laser USA with a flat, metal handle & flat tape sheath perhaps?
Some handy links with good selections of spearo knives:
http://www.bluewaterhunter.com/shopsite_sc/store/html/shop_knives.html
http://www.scubasworld.com/products.php?cat=13