Seawolves are an extremely odd species of therianthrope from the worlds of Dungeons & Dragons and can literally be described as "what if werewolves could be found at sea?" No, seriously; they're literally the aquatic equivalent of werewolves. And now you know all there is to know about seawolves.
...Okay, we'll elaborate. Seawolves are an aquatic strain of lycanthrope; we don't know if they diverged from their land counterparts, or if this means there's also a land seal. They can assume the form of either a humanoid wolf with waterproof fur and webbed digits, or the form of a huge wolf-headed seal. In AD&D, there were two distinct strains of seawolf; the Lesser Seawolf is smaller (6-7ft long) and has none of the traditional monstrous resistances, whilst the Greater Seawolf is huge (12-15ft long in wolf-seal form, 6-7ft tall in human or hybrid form) and resistant to damage not delivered by enchanted, silvered, or cold iron weapons. This split was lost in 3rd edition. Much like their land-dwelling counterparts, seawolves are typically marauders who roam the ocean, harassing shipping and sailors where they encounter them; many operate as pirates and raiders, and even the most "peaceful" of seawolves tends to gravitate towards oceanic hunting vessels. The AD&D version is far more malevolent and destructive than its 3e counterpart, but even the 3e version isn't exactly a fluffy bunny!
Seawolves debuted in 1982's Monster Cards, and then made it into the Monster Manual II for Advanced Dungeons & Dragons 1e. They received a 2e update in the Monstrous Compendium Annual Volume One, and from here made it to the 1993 Monstrous Manual. Their final appearance was in Stormwrack for Dungeons & Dragons 3rd Edition; whether it's because aquatic adventures are so unpopular or it's because the idea of "aquatic werewolf" is kind of hokey, they haven't appeared since.
The seawolves are humans who can assume a form combining aspects of a seal and a wolf. Their packs roam the seas in search of ships to attack.
The monstrous form of the lesser seawolf has the 6 to 7 foot long body of a seal. The head and shoulders are those of a wolf. In human form the lesser seawolves stand 5 to 6 feet tall. All are thickly muscled and have tiny ears and long hair that covers their head and shoulders like a mane. The lesser seawolf has a hybrid form of a wolfman, a humanoid shape that retains the seawolf’s teeth, claws, and fur; statistics for this form are given in parentheses above.
Combat: Lesser seawolves approach a ship in seawolf form, then change into the hybrid form and climb aboard. There they use their teeth and claws to kill their opponents. If the ship looks too heavily defended, the seawolves may gnaw holes in the hull in order to sink the ship.
Unlike most other lycanthropes, lesser seawolves have no special protection against normal weapons. Dead seawolves revert to their human form in a single round.
Habitat/Society: Most seawolves were formerly fishermen or sailors; as such, they also tend to be male and human. They travel in packs with those of their own kind. Their fierce hatred of their former coworkers drives them to seek to kill them or pass on the lycanthropic curse. Victims who acquire the disease become seawolves in 2-5 days. Once night falls, the new seawolf slips into the water and goes off in search of a pack.
Seawolves are nomads constantly roaming the cooler sections of the seas. They neither build lairs nor keep treasure. During the day, they sleep on beaches or in caves or appropriated houses. If surprised on a beach, they pretend to be shipwreck victims, then kill the intruders and take their clothing. If at sea, the seawolves are still able to sleep during the day by floating on their backs; in this case they may be mistaken for a cluster of drowning victims. If a ship moves close to investigate, the seawolves wait for the best opportunity to attack and take over the ship.
Seawolves breathe air. They can remain submerged for 17-24 (1d8+16) minutes. Failure to surface after that time causes them to suffer 1-6 points of damage each round until they drown.
Female seawolves give birth to single cubs. These may appear to be human infants during the day or baby seals at night. Seawolves lack parental feelings and abandon the cubs. Although the cubs are able to swim and hunt from birth, they have difficulty keeping up with adults and often drown during the day if they are at sea. Only 5% of cubs reach adulthood. The offspring of a seawolf and a human are good swimmers who feel mysteriously drawn to the sea, but few (25%) become seawolves upon reaching adolescence.
Their diet includes a variety of foods, such as shellfish, fish, seabirds, sea mammals, and anyone they can sink their teeth into. Occasionally, packs may wander into a town and take over a tavern for a round of drinking and wenching.
Seawolf personalities are a twisted version of their original, human personalities. It is as if the seawolf persona is a savage, magnified version of all the original person’s bad traits. Seawolves periodically return to their original home port. This may be a subconscious longing for their old life or a means to renew their hatred of those still humanoid. If a seawolf spots his old self’s mate or child, he may attempt to make contact.
Ecology: Seawolves are the sworn enemies of any humanoid who makes his living in the sea. They live to destroy shipping, spread terror, and spread their curse further.
Greater Seawolves The monstrous form of the greater seawolf has a 12 to 15 foot long body, but is otherwise identical to the lesser seawolf. In human form, greater seawolves stand 6 to 7 feet tall.
Combat: Greater seawolves assume their human forms to get close to their opponents. The typical plan is to bite or strangle one or two deckhands, take their weapons, and begin a general assault. In seawolf form, greater seawolves can be harmed only by silver, cold iron, or magical weapons of +1 or better. Steel weapons have no effect. Dead seawolves revert to their human form in one round.
Seawolf packs hunt ships at sea, climbing aboard in the dark hours of the night to kill all aboard and seize any treasure they can find.
COMBAT: Seawolves are cunning and sly. They use their human forms in order to set others at ease but prefer to fight in their seawolf or hybrid forms. A common seawolf trick is to insinuate one or two members of a pack into a ship’s crew. The spies then steer the ship to the waiting pack, or murder the ship’s watch in the middle of the night so that their fellows can clamber aboard unopposed.
SEAWOLF SOCIETY: Solitary seawolves often pass themselves off as pirates, raiders, or seahunters, where they usually earn the fear and respect of their fellows due to their violent temperaments and unusual strength and hardiness. By nature seawolves are social creatures and, given a choice, gather in small packs in order to cooperate in their raids and attacks on humans and humanoids. Seawolves often keep a secret lair somewhere on a shoreline, where they hoard the treasure they’ve stolen from ships they attack. Seawolf packs have been known to adopt human guise and venture into coastal towns for a night of eating, drinking, wenching, and fighting. Quick to draw blades or go on a murderous rampage, the seawolves are given a wide berth by those locals who suspect their true nature.