“So what’s the assignment, again?”
“A wereshark. Like a werewolf, but a shark.”
“So is it like a special-looking shark or something?”
“No, it looks like a regular shark, though the art has to express the ‘were’ part. You’re smart. You’ll figure it out. Oh, and I need it on my desk at eight tomorrow morning.”
I can easily visualize a decent adventuring scenario involving weresharks — a tropical spin on the isolated and not-quite-right village trope with a bit of the ol’ Innsmouth vibe thrown in for good measure — but the creatures’ limitations as a threat are also quite apparent.
Land-dwelling lycanthropes have more latitude when it comes to stalking and harassing player characters. The nocturnal nature of their curse and the arboreal environments they haunt make it possible to set up tense scenarios where something is lurking in the shadows, toying with its prey and using the characters’ panic to herd them towards the anticipated slaughter.
You don’t really get that with weresharks, though. Even if the player characters were of an aquatic bent (which none of my groups ever were), any physical threat would have to involve getting them out onto deep enough water during the dead of night. Any characters that would take that blatantly obvious bait would’ve met their demise long before reaching a level where a wereshark would be a fair challenge.
Otherwise, all they have to do is head inland to negate the threat.
This also brings up the mechanics of a wereshark’s transformation. They’d somehow need to arrange to be a few yards from the shore when the curse takes effect. What happens if they get caught out on dry land?
Maybe that’s what happened to the tragic-looking creature in the Monster Manual II entry. He was out with his buddies in a neighboring village, drank too much of the local hooch, and lost track of the time. Now he’s stuck flailing about half a mile from the lagoon, slowly suffocating, and remembering the time when he made fun of that pack of weremoles for being a bunch of near-sighted wussies.
Recommended listening: Eddie Duchin and His Orchestra – Between the Devil and the Deep Blue Sea (from a 1938 single)
Sink or swing.