For this to make complete sense, you’ll need to know what Grims are. This is my info on them, and it’s quite long, but you only need to read the first part (on Grims in general) and maybe the part on Drowned Grims. 

https://bettsplendens.tumblr.com/post/159159118849/grim-beastiary

(feel free to message me about these three, I will happily yell about them)

Sebastian was born somewhere around 1720, and drowned in a burlap sack somewhere in the 1740s for supposedly being something along the lines of a changeling. His black dog was drowned with him, mistaken for a familiar and too loyal to save herself, and soon after their bodies settled into the sea floor she had transformed into a Grim. Somehow, through loyalty or determination or maybe just from how closely entwined they were, Sebastian went with her and became the only known formerly-human Grim. 

No one above water saw Sebastian for at least ten years. When he literally re-surfaced, he brought a ship with him, an ancient thing with tattered sails and no name carved anywhere on it. 

The ship has no figurehead, but its prow has eyes carved along it, eyes like those of whales. The sails look wet and are shredded in places, the entire exterior of the ship is speckled with barnacles, and the wood looks as though it has been submerged for centuries. Nothing is visibly rotten, but everything looks old and wet, and nothing seems to break down. 

When asked why the ship has no figurehead, Sebastian simply points down, leaving them to find the answer for themselves. The ship has no figurehead because it can watch the way for itself, as its frame is built along the back of a whale. More specifically, a whale skeleton, larger than that of any known whale species, with the ship fused into its backbone and the belly of the ship around its ribs. 

Its skeletal fins and tail are filled out by wood and canvas, and, though it moves slowly most of the time, it provides most of the power to move the ship. Its skull reaches out ahead of the ship, its tail behind, and part of the hold flexes slowly with every motion of its tail. If the whale speeds up, it can carry the ship faster than any other sailing ship can move, folding the sails and outrunning the wind itself. 

The ship can dive at will, folding the mast back and the supporting structure of the sails inward to provide less resistance, and anyone who has permission to be on the ship can breathe just as easily as if they were in clean air no matter how far they are underwater. Anyone on the ship against Sebastian’s will is left floating as the ship sinks. 

The ship takes orders from Sebastian when he asks, but anyone else’s hands on the wheel will be greeted with angry, ghostly cries and the ship attempting to buck them away. If he goes on land and leaves the ship, it dives and swims off, only coming back when he touches saltwater again.

It has a name, but, as the only two beings who know the name don’t talk, no one else is ever going to hear it. 

Sebastian himself looks somewhat unremarkable most of the time. He looks somewhere around mid-teens, with black hair and pale skin. He usually wears a hooded coat of some sort, always with the hood up, and has tall black boots with silver buckles. He keeps his gaze low most of the time, but, if he looks up and can be convinced to keep eye contact enough to get a look, his eyes are an odd shade of silver-grey. Little to no expression on his face at all times, at least one fidget toy in his pockets, often something like a smooth ball bearing in one hand, always with the dog somewhere near him. 

As I said earlier, Sebastian is autistic, and nonverbal. He tends to stim quietly, little motions held close to his chest, but gets more expressive if he trusts the people he’s around. If he starts flapping or bouncing visibly, that’s a good sign, it means he’s comfortable around you, but he doesn’t tend to be too expressive even when he is comfortable. A quiet person, in every sense of the word.

The dog has a name, but, like the ship, no one is ever going to hear it because Sebastian is the only one who knows it. She normally looks fairly unremarkable, just a medium-sized, slightly shaggy black dog with a long snout. The oddest thing about her is her eyes- an unremarkable golden-orange in color, but intense. She watches everything with a steadiness more like the gaze of a cat, observing, seemingly comprehending. Because she is- she wasn’t at first, but she’s a Grim, a few hundred years old at this point, and she’s a sapient, intelligent being. Which is a bit creepy when you aren’t aware of that fact and a dog is staring at you like it knows what you did. It gets creepier if you catch her lying flat on her side and realize that she isn’t breathing.

When the situation allows for it, though, she goes from slightly creepy to outright horrifying. All Grims have a natural form much larger than their guise, and hers is no exception- easily the size of a draft horse, dripping seawater, with kelp and tattered, rotting burlap dangling from her fur and sessile invertebrates like barnacles growing from her flanks. Her eyes become blank white orbs, she drools water whenever her head is lowered, and the deck of the ship creaks under her weight with every step. 

She runs faster than most ships can sail, and, being a Drowned Grim, she can run across the top of the water with no effort, flip over to run with her paws on the surface of the water and her body underwater, or dive and run at any depth she wants with no additional difficulty. She generally stays in her guise in order to stick close to Sebastian on the ship, there isn’t enough room for her in most areas otherwise, and she takes her full form whenever they leave the ship and don’t need to pretend to be normal people. 

Sebastian, being a Grim, does have a larger ‘natural’ form. He doesn’t usually wear it, as it tends to freak people out, but he has one. Easily two feet taller than his guise, a hulking figure who walks in slow, swaying steps and, rather than looking at nothing in particular, tends to lock his eyes on people who are now much shorter than him. 

His skin goes from pale to translucent, as does the muscle below, faintly showing his skull and the skeleton in any parts of his body not covered by clothing. He wears something that looks like an old, tattered sea-captain’s cloak, torn at the edges and constantly soaking wet, and has seaweed and shreds of canvas dangling from his frame. His boots are heavy and full of water, his hair grows down to almost mid-torso, and his eyes go the same milky-white as his companion’s. 

Sebastian can push away the water from his frame and stop dripping if he really needs to in this form, but he finds it unpleasant. This form is sometimes used for intimidation and whatever fights he can’t avoid, but he prefers to use it for things more playful- standing on the bow of the ship during a storm, channeling his abilities to pull up a huge wave for him and his companion to ride, or moving more easily along the bottom of an interesting patch of water. He likes to snoop around in shipwrecks, and coral reefs are fascinating. 

As Drowned Grims, Sebastian and his companion have aqua-kinetic abilities. They can pull up and channel waves, and Sebastian has shown a knack for summoning storms and even drawing lightning strikes. He may have had some abilities before his death, which could have assisted his transformation, but no one is ever going to know because Sebastian isn’t inclined to share. He’s shy and has trouble trusting people.