All I have to say is three things. 

One: Get ready to have a biology dump/rambles. 

Two: This AU is just for fun. You can choose to take it seriously or for fun. 

Three: There is no time period this AU happens in. Maybe just while the Strawhats travel the seas.

Now onto the AU lore and design details. 

During a fight with some other pirates, one of the enemies had hit Jinbei with a knife. The Strawhats did defeat the enemy and left to check on the wound from the knife. Chopper checked on the wound and found it was a normal wound from the knife. He did test the samples from it and found nothing unusual. So, they treat it like a normal wound. 

They were unaware that the knife had a tiny drip of a mystery liquid on the tip. It was clear as water. The drop made it to the blood stream before being caught. Strawhats were unaware of the nightmare that would soon come down on them. The first two days were normal for the helmsman. He felt normal and nothing bad. 

However, day three was different. 

Jinbei noticed how tired he was getting, craving something raw, mind getting fuzzy, and how he felt his body temper rising a little. Chopper suggests it could be a fever or possibly an infection. The reindeer told him to rest for some days until it gets better. The helmsman agreed as he didn’t feel the power to try to argue about it. 

He was checked on by the other members. Luffy was worried but tried to cheer up the whale shark. Franky has taken over the ship while Jinbei is out. He always checks on the fishman during breaks. Sanji brought food and tea to help Jinbei get better. Zoro checked in after everyone else did. He knows the whale shark will get better because he is strong. 

Nami and Robin both check in. They break any extra blankets, pillows, or anything he may need. Usopp checks in and asks if there is anything he could get for the boss. Brook offered to play any songs that could make Jinbei comfortable while recovering. The helmsman was happy to have them helping him but also a little guilty. 

During the night of day four, Jinbei woke up for some reason. He decided to go outside to get some air. The fishman removed the bandage on the knife wound. He froze at the wound. It was a dark purple color and had dark purple veins coming out of it on his skin. The wound wasn’t like that when Chopper checked on it before everyone went to bed. 

Then the real horror began, it was his first transformation into the monster the mysterious knife put into him. It was bloody and painful to his body. Jinbei black out from the pain during it. 

The next morning, Jinbei woke up with no memory. His kimono was ripped. Only the bottom part of the kimono was still in at least one piece. It just has some rips to it. Also some blood on it. Jinbei noticed his hair was down and messy. His hands and face were covered in dry blood as well. Horror and fear wash over him. 

It took him a bit to realize the crew was near and just looking. A mix of confusion, fear, and worry was on their faces. Jinbei asks what happened. 

That night of the transformation, the crew woke up to their helmsman letting out a scream of pain. They all quickly rush out to help but only find the horror. The whale shark had turned into a monster. 

Luckily, the beast jumped into the water and didn’t attack the crew. The Strawhats did follow the direction the beast swam to. They did catch up to the animal but they weren’t ready for when they did. The beast was fighting against a Sea King. Some chunks were missing from the king. The crew just watched in shock and horror at the scene. 

After the fight, they watch as the beast snack on the chunks it ripped out of the Sea King. The king ends up swimming away after taking its damage limit. The Strawhats use this time to come up with a plan to catch the beast. All they could come up with was this: 

One: Get a chunk of the meat that it was feeding on. 

Two: See if the beast would come aboard the ship. 

Three: Chain it down to keep it from attacking the crew. 

So, they put the plan into motion. They get the beast to come to the Sunny. When it was step two, the beast came aboard with no issue. The beast ate the meat on the ship deck with no issue. The crew quietly gets ready for step three. 

Then the crew hear Luffy laughing and look over to the direction. The Strawhats froze in both shock and maybe some fear. Their captain was petting the beast like an oversize cat. The beast was letting him be the shocking part.

It was attacking a Sea King seconds ago and now acting like an old cat. The crew decide to not chain the beast for the moment. They wanted to see if the beast was going to be friendly to them. 

When the other crew members slowly walked up to the beast, they got the same behavior their captain got. The crew talk about what they saw and how short this happened after the fight with the other pirates.

They knew this beast was their helmsman. They saw him during the halfway transformation. It was a nightmare and a horror scene to see in person. Now they need to find what is going on and how to fix it. They want to help Jinbei, their friend. 

What was the liquid that was on the knife? 

The enemy made a disease that changed a person to a wereanimal version of a Sea King. However, it only works on fishman when Jinbei gets infected. It was very new at the time of the fight. Their plan was to infect the helmsman so that when he turns into the wereseaking, he attacks and ends his own crewmates.

It was tested and it was a success for the enemy before they used it on Jinbei. They were unaware of the strong will and memories that Jinbei had. Those two things keep the helmsman from being the monster that the enemy wanted. 

The disease has a time limit to be cured. It is 7 months. A lot of time to try to find a cure but also time flies fast while searching sometimes. The dread of time dying down on getting rid of the Sea King Disease.

During this time, Jinbei would fight within himself about this form. He hated how he was turned into a monster. Even if he doesn’t attack the crew, he won’t know until he wakes up to see them alive. 

This AU has two endings for you guys to pick for yourself. You can pick the good ending or bad ending. I won’t pick an ending because I like both endings. 

Good ending: The Strawhats end up finding a cure for the disease and help Jinbei get rid of the beast. 

Bad ending: The crew couldn’t find the cure before the time limit. The wereseaking form end up staying forever. 

How do the forms interact with the crew? 

Luffy: treats him like a Sea King pup. Let Luffy ride on his back or head. Very protective of him. 

Nami: careful with her. Have some fear of her wrath/anger. Scared of hurting them (or any of the crew members really). Treats her like a sea King Pup sometimes. 

Zoro: takes naps along with the swordsman. Try to sneak a drink of sake or other alcohol. Watch him train on the deck. 

Sanji: likes the food he makes. Often bring him the prey he caught for the chef to cook. 

Chopper: treats him like a Sea King pup as well. Allows the doctor to do check ups. Watch Chopper take notes about himself. 

Usopp: treats him like a scared Sea King pup (the form does this to almost all members in some form). Gentle with him. Try to help with catching fish. 

Robin: Let her lay against him. Read whatever book she is reading. Let her and Nami brush his messy hair and do some styles. 

Franky: Let him try to build a saddle to put on the beast’s back for the crew to hold onto. Like to watch him work around the ship. Try to help with anything with the ship like letting Franky sit on his head while working on the sides of the Sunny. 

Brook: try to chew on his bones at first but stop later on. Loves his music, especially the heavy metal ones. Just like sharks. Sometimes confused by the skeleton’s ability to run on top of water. 

Is the transformation painful? What inspired the design? 

Yes, it is very painful and bloody. Jinbei’s body is undergoing a lot of changes to his body. Like size and having new features. The dermal denticles on his skin have to grow bigger for his new size. Bigger and sharper teeth. Bones breaking and reforming for his Sea King body.

I already drew Jinbei with a tail in my style, so his tail just got bigger and a bit longer. Same for claws. First signs of the transformation is coughing of blood and a nose bleed. The change of his iris color. 

I kept his eyes purple like his 30s self for both normal and beast form. The Sea King form is a more violet or darker purple shade. I used the color manga for Jinbei’s colors. Shark biology for this is through the roof.

(I could go for months to talk/ramble about it but I won’t). I took some ideas from a japanese dragon and Oni (demon). Fins are on the back legs but are a bit smaller than the forearm ones. The kimono bottom half stays on by “magic”. 

I see Zoan Devil Fruit users somehow keeping their clothes on, so I just use that for him. 

His markings have changed in this form. The dark blue have form stripes like a tiger at the bottom of it. White lines that are normally straight were now lighting like. Some parts of the lines go down to the dark blue stripes.

I wanted to make him look a little different but still himself. He gets another set of smaller versions of his tusks. Also some large fangs as well. Jinbei has a lizard-like tongue or the same tongue design as Toothless from HTTYD. 

The wereseaking form does feed on the dark part of the helmsman’s mind. This is inspired by the drawing of the inside of Jinbei’s mind that Oda did. It feeds on the rage that builds up over the years from his past. However, it doesn't lash out at the Strawhats for the rage.

The Sea King lash out at everything else though. Especially on Marines (due to the memory of Fisher Tiger’s death by them). The memories that are broken in the beast’s mind keeps the beast from attacking the crew. Both good and sad memories. 

It's like the memories are showing the wereseaking who to trust and not to trust. Jinbei doesn't have any memories from when he was in the form. He hates it.

Regardless if he isn’t dangerous to the crew, the helmsman still feels awful about having this monstrous form. He does have the ‘teeth' around his eyes like a Whale Shark. 

I used Jinbei’s image animal to add to the Sea King’s behavior. I chose a Kodiak Bear for it. I do have a side project of my take on Fishman that is taking a long time due to trying to find examples for parts of it.

Like how Jinbei’s teeth are just a large version of the small ones that Whale Sharks have. They are hook-like and kind of a little bit like Tiger Shark. The form is big as an Ichthyotitan (shown in the design sheet). I used the design of the werewolf from American Werewolf in London. 

That is all I have for the moment. I’m sorry if it is rambly and seems like a crazy person talking. Before this post ends, I just want to say this. 

Fanart, fan animation, writing, etc is always allowed. Just tag me. If you’re nervous or uncomfortable tagging me in it, just use the tag “Sea King Disease AU”. I love Jinbei with my heart and it does hurt to add more angst to him. Questions are welcome.