Is the fish laterally compressed (flat in a vertical direction) or dorsal-ventral depressed (flat like a catfish)?
I know there is a whole family of weakly electrical fish from Africa and South America that are being called by strange trade names. There is the "baby whale" which has the round, cigar or bullet shape you are describing but is usually brown, the "baby dolphin" a similar fish with a beak, the "elephant nose fish" which I'm sure all of us have seen at some point in the LFS, and of course the "Black Ghost Knife." These are honest freshwater fish from blackwater, mucky environs that don't have good eyesight so they use a weak electric field to find prey in the silt, which is primarly worms. Many of them are still very new to the trade and are primarily wild caught. I don't think all species have even been classified yet.
So far as I know though, all true sharks are saltwater sharks. Even though there are freshwater rays, there is no such thing as a freshwater cartalagionous shark that I am aware of. Sharks are known to go far upriver into freshwater (as in the case of bull sharks found 300miles up the Mississippi), but they spend much of their primarily life in the ocean. Being in true fresh water for an extended period of time is very stressful to many species of shark, especially your oceanic species, even under proper acclimation. These creatures cannot maintain the bouancy they need, nor the proper osmotic pressure in their blood, which may explain the awkward swimming style. True sharks swim on a level plane, they have to, to take advantage of the lift provided by their pectorals.
~~Colesea