Since there's a direct no for Sharks why not meet the Shark crowd in the middle with a Dragon/ aquatic lizard creature hybrid. Maybe multiple extra fins or a double jaw (Slifer style) and some bright skin pigmentation.
Since the association with sharks is unwanted I assume mob behavior cannot be like that of a Shark. In which case increasing activity of the mob at night or "feeding frenzies" should be avoided. Alternatively this new mob could avoid each other if sufficient room is available. Or fight one another like Chinese Fighting fish. They could vary in size depending on how many battles they've won against each other, fights wouldn't need to result in a mob death they could just have a percentage to flee if losing.
As for a purpose they could exclusively seek out Drowned or random fish similar to Wolves behaviour towards Sheep.
Granted I'm no Shark expert so if similar mob behavior is undesirable I'd probably double check, for all I know they probably brawl all the time.
This would probably fill the Shark void some people have and maintain the all hostile mobs are fictional thing. Might be a bit messy but it's just a thought to maybe make both sides happy.
I think Subnautica can be a good source of inspiration if one is trying to avoid Sharks. Another thing could be to use those monsters that were drawn on old maps, huge sea snakes and "sea boars" or things like that. Alternatively, sirens.
I am under the impression, however, that the shark addition supporters want the beastie specifically for its looks and/or its behaviors – nerfing both seems unlikely to satisfy.
Swapping in a reskinned shark ought answer MS/Mj's 'child endangerment' argument. [My preferred design for Not-a-Shark [NaS] could be described as a squid mouth (sans tentacles) on the front of a living torpedo (cruiform tail both to avoid 'sharky-ness' and distiguish from dolphins) with a color shifted magma block skin to make it more camoflaged in its native ravine environment. ]
(Whether such a tactic could defeat the stomp the feet and shake the curls rejection MS/Mj has deployed is another matter.
As is whether such a beast would be sharky enough to satisfy.)
Size variation in mobs seems like it would be quite limited due to the comparatively large block size in MC (cf. slime and magma cubes)
As an option to a straight hostile mob, the NaS could be quite powerful but neutral unless attacked (following the polar bear model).
Leashable/cagable [a longstanding bug makes leashes useless if the chunks load/unload] but not tamable or breedable.
If the Nas would attack drowned (highly useful to the player and offering the glimmerings of an effective means of warding a water base), keeping them ought (for purposes of game balance) to have some downside. Attacking fish and swimming mobs (including dogs, cats, parrots, and villagers) as well as 'eating' (ie breaking) kelp (one up from the base) might be enough to make having a pack patrolling one's waters was not an unmitigated benefit.
[Potentially, a very limited ability to attack mobs at the edge of water might be added, but that might present coding difficulties.]
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WARNING: I have an extemely "grindy" playstyle; YMMV — if this doesn't seem fun to you, mine what you can from it & bin the rest.