Originally, the wereshark only had two forms: the humanoid form and the shark form. It gained a hybrid form in 3rd edition, though if I were to use one, I'd probably stick to the two forms. The wereshark detailed here is based on existing D&D material. The Razor Coast campaign from Frog God Games has a slightly different take on the wereshark (spoilers). They're still arrogant and cruel, but they are less individualistic due to being dedicated to an evil shark god. So you end up with groups of the darn things, and they do have the hybrid form.
Now on to the stats:
The original MMII picture.
Medium humanoid (human, shapechanger), neutral evil
Armor Class 11, 12 in shark or hybrid form
Hit Points 97 (15d8+30)
Speed 30 ft. (0 ft. in shark form), swim 40 ft. in shark or hybrid form
Str Dex Con Int Wis Cha
18 (+4) 12 (+1) 15 (+2) 10 (+0) 11 (+0) 9 (-1)
Skills Perception +2
Damage Immunities bludgeoning, piercing, and slashing from nonmagical weapons that aren’t silvered
Senses blindsight 30 ft. in shark or hybrid form, passive Perception 12
Challenge 4 (1,100 XP)
Amphibious (shark or hybrid form only). There wereshark can breathe air and water.
Blood Frenzy. The wereshark has advantage on melee attacks against creatures that don’t have all of their hit points.
Shapechanger. The wereshark can use its action to change into a Large shark-hybrid or Large shark form, or back to its true form, which is humanoid. In shark-hybrid or shark form, it gains a swim speed of 40 ft., and loses its 30 ft. speed in shark form. All other stats, other than its size and AC, remain the same in each form. Any equipment it is wearing or carrying isn’t transformed. It reverts to its true form if it dies.
Multiattack. The wereshark makes two attacks, only one of which can be a bite.
Bite (shark or hybrid form only).Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 13 (2d8+4) piercing damage. If the target is a humanoid, it must succeed on a DC 12 Constitution saving throw or be cursed with wereshark lycanthropy.
Boarding Axe. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 7 (1d6+4) piercing or slashing damage or 8 (1d8+4) piercing or slashing damage if used with two hands.
A character cursed with lycanthropy gains a Strength of 18 if his or her score isn’t already higher, and a +1 bonus to AC while in shark or hybrid form (from natural armor). Attack and damage rolls for the natural weapons are based on Strength.
Weresharks are avaricious brutes, individualistic and always out for their own gain. As humanoids, they are muscular, frequently showing evidence of a shark attack in the form of scars or possibly even a missing limb. These old wounds are not evident when in shark form. Arrogant and cruel, weresharks are also fiercely territorial and will defend a claimed area to the death. The only creatures weresharks will usually associate with are real sharks, though it’s not unknown for a wereshark to cooperate with sahuagin or a priest of an evil sea god.
A wereshark fighting on land will use weapons appropriate to its previous maritime existence, such as a boarding axe. It much prefers to do its fighting in the water, where it has an advantage. Weresharks don’t tend to worry about infecting victims with lycanthropy. It’s a rare occurrence for a wereshark to leave a victim alive, so it rarely becomes an issue.