Vampire Shark

Blood Frenzy. The vampire has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Limited Amphibiousness. The vampire can breathe air and water, but it needs to be submerged at least once every 4 hours to avoid suffocating.

Regeneration. The vampire regains 10 hit points at the start of its turn if it has at least 1 hit point and isn't in sunlight. If the vampire takes radiant damage or damage from holy water, this trait doesn't function at the start of the vampire's next turn.

Shapechanger.  If the vampire isn't in sunlight, it can use its action to polymorph into a Medium humanoid form with sickly, reddish skin and a multitude of shark-like traits, or back into its true form.  It is considered undead in either form.  Its statistics, other than its speed, are the same in each form.  Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Vampire Weaknesses. The vampire has the following flaws:

Forbiddance. The vampire can't enter a residence without an invitation from one of the occupants.

Stake to the Heart. The vampire is destroyed if a piercing weapon made of wood is driven into its heart while it is incapacitated in its resting place.

Sunlight Hypersensitivity. The vampire takes 20 radiant damage when it starts its turn in sunlight. While in sunlight, it has disadvantage on attack rolls and ability checks.

Multiattack. The vampire makes two attacks, only one of which can be a bite attack.

Claws. Melee Weapon Attack: +6 to hit, reach 5 ft., one creature. Hit: 5 (1d4 + 3) slashing damage. Instead of dealing damage, the vampire can grapple the target (escape DC 13).

Bite. Melee Weapon Attack: +6 to hit, reach 5 ft., one willing creature, or a creature that is grappled by the vampire, incapacitated, or restrained.  Hit: 7 (1d8 + 3) piercing damage plus 7 (2d6) necrotic damage.  The target's hit point maximum is reduced by an amount equal to the necrotic damage taken, and the vampire regains hit points equal to that amount. The reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.  A humanoid slain in this way and then buried in the ground or at sea rises the following night as a vampire spawn under the vampire's control.  A shark slain this way rises the following night as another vampire shark under the vampire's control.