Wadatsumi is an enormous fishman, equal in size to the head of the kraken, Surume. He is large enough to make the giant-sized Shirahoshi and Megalo look like a regular-sized mermaid and shark. His round head and chest are quite big in comparison to his arms and legs. His hands are webbed. He has a big, round face, with a large mouth full of massive, round teeth quite distant from one another, and child-like eyes. He has some stubble on his chin and dark hair surrounding his head, taking the shape of a protrusion pointing upward at the top, which also covers a part of his shoulders. His whole body is also quite hairy.
He apparently wears a pair of extremely loose pants paired with a dark indument tied around his waist, and has been seen walking barefoot.
His overall shape, size, and eyes strongly resemble that of Sanjuan Wolf's. Due to his appearance, Wadatsumi was incorrectly called an Umibozu by Usopp. Members of the New Fishman Pirates also refer him as such.
He is loyal and obedient, following Decken's every command without question. Despite his huge size, his personality appears to be childlike, mirrored by his extremely childlike voice in the anime. Believing Decken has abandoned him, Wadatsumi cried and complained, but recovered when he was deceived into thinking he misheard, showing that Wadatsumi is naïve and gullible.
Contrary to the nasty reputation and behavior of his crew and captain, Wadatsumi seems to be more of gentle and kind, as he destroyed two towns only due to sheer carelessness, which he apologized for in a genuinely remorseful manner, and cheerfully celebrating with citizens upon fixing the situation.
Abilities and Powers
Due to his great size, and the fact that he is a fishman, he is extremely powerful. He shows his strength by hurting Ankoro, an angler fish several times the size of the Thousand Sunny, with a single punch, even though the punch was only intended to prevent the fish from eating the Straw Hat Pirates, and not to harm him. Also, living in the depths of the Grand Line, he is capable of breathing underwater as well as surviving the crushing pressure of 10,000 meters below sea level. Being a tiger blowfish fishman, he's also capable of inflating himself, thus increasing his already enormous size.
After taking an Energy Steroid, his great strength increases further by being able to defeat Surume with one blow. He also has high stamina, taking powerful strikes from Jinbe and Sanji respectively and only slightly complaining about the pain.
- Mega Nyudo (メガ入道 Mega Nyūdō?, literally meaning "Mega Monk"): Wadatsumi inhales a large amount of air, and expands his stomach to enormous proportions, crushing anything in his vicinity. If needed, he can expand even more, furthering his size. However, he must hold his mouth to prevent the air from leaking out, and if he wishes to talk, he must speak in a fast pace to prevent air leakage. As he says, in this form he attacks by rolling into the battlefield, crushing all enemies in his way. This was first seen used against Sanji, though he indiscriminately crushed his New Fishman Pirates allies. This is called Mega Monk in the Viz manga and FUNimation sub. 
Fishman Island Arc
While the Straw Hats (minus Luffy, Zoro, and Sanji) were sailing through the underworld of the sea, Wadatsumi suddenly appeared from the rocks beneath the sea. He beats an angler fish with one blow to prevent it from eating the Thousand Sunny because the captain of the Flying Dutchman, Vander Decken IX, wants their treasure. On Vander Decken's orders, Wadatsumi attempts to punch Thousand Sunny, but the kraken (which had been tamed by Luffy, Zoro, and Sanji) intervenes and knocks out Wadatsumi.
When the volcano erupts, Vander Decken orders Wadatsumi and Ankoro to pull the ship away from the magma. As the water temperature rose, he and the Flying Dutchman were thrown forward.
He is later seen walking on the sea floor pulling the Flying Dutchman. Vander Decken IX asks how many years it has been that the Mermaid Princess has not replied to his letters. Wadatsumi answers that it has been ten years and the princess still has not replied, enraging Decken. He is then seen in Noah in the Fishman District where Vander Decken IX and Hody Jones form an alliance.
After Luffy takes down Decken at Coral Hill, Decken calls out to Wadatsumi to prevent Princess Shirahoshi from escaping. Before Wadatsumi attacks, Luffy beats him back with a Jet Pistol, breaking off one of his teeth. Wadatsumi cries out in pain as Shirahoshi, Luffy, and their friends escape.
Wadatsumi is then seen back at the Fishman District and later running away, deeply shocked that his captain "threw" Noah toward Shirahoshi.
When Noah and the Flying Pirates arrive to Gyoncorde Plaza, Wadatsumi falls into the plaza and begs for his life to Vander Decken IX. Decken replies that he cannot stop the boat anymore and that he would become a necessary sacrifice for the end of Fishman Island. Fortunately, the boat goes after Shirahoshi, who had just left Fishman Island to sacrifice herself.
Wadatsumi then gets beaten up by Surume. Saddened by Decken's betrayal, the giant has no will to fight back. Zeo talks Wadatsumi into fighting and gives him an Energy Steroid. Now stronger than before, Wadatsumi defeats Surume. He then tries to attack Jinbe, but he manages to deflect his attack, causing Wadatsumi to fall to the ground. Sanji then kicks him in the head, forcing the giant to stand up. Wadatsumi then fights against Jinbe and Sanji. Regardless of his size, Wadatsumi feels pain from their attacks.
Having a difficult time, Wadatsumi starts inflating himself, growing to a size that doubles his normal one. Sanji, standing on Wadatsumi's cheek, is not at all afraid of Wadatsumi's new form and starts mocking him, telling him that Surume is still bigger than him. Wadatsumi falls for it and sucks even more air, keeping on growing and crushing many Fishmen Pirates in the process, with Sanji still standing firm with his previous statement.
As Luffy pummels Hody and Noah, Sanji is seen standing on an almost-spherical Wadatsumi, who keeps asking whether he's surprised yet. While obviously neither him nor Jinbe are, they decide to execute their unseen plan.
Jinbe surrounds his right hand in water and hits Wadatsumi with Buraikan, which enters the giant on one side and explodes out of the other side. Wadatsumi begins rolling away, but as he cannot hold his breath any longer, he releases the air he took in and blasts himself away. Sanji follows Wadatsumi into the sky. Sanji then unleashes his rage by thinking back to the torture he had to go through during the last two years. Sanji engulfs his whole body in flames and kicks Wadatsumi with an attack called Hell Memories, a fire-attack so powerful it completely scorches Wadatsumi. With that, Wadatsumi is finally defeated. The unconscious Wadatsumi then fell on his unfortunate comrades who did not flee the plaza in time.
After the battle, a still unconscious Wadatsumi is banished from Fishman Island (because he is too big to fit in any prison they have), and is seen being carried away by Surume to the far reaches of the ocean.
Solo Journey of Jinbe, Knight of the Sea
Some time later, Wadatsumi traveled to a port town and posed as a sea god. In return for the offerings given to him, Wadatsumi raised up the homes at the bottom of the sea. He encountered Jinbe again while the former Shichibukai was investigating the disappearance of a Sea Kitten's home. Having destroyed two towns due to his carelessness, Wadatsumi earned himself a scolding from Jinbe. Wadatsumi and the Sea Monsters later apologized to the towns people for the trouble they caused. After they made peace with each other, Wadatsumi, Jinbe, the locals, and the Sea Monsters decided to have a feast together. On the next day, Wadatsumi asked Jinbe if he can come along with him. After bidding the townspeople and the Sea Monsters farewell, Wadatsumi left with Jinbe.
Totto Land Arc
Wadatsumi was with the Sun Pirates while Jinbe was discussing with them about his intention to leave the Big Mom Pirates and join Luffy's crew. Jinbe apologized to him for coming such a long way but Wadatsumi states it was alright. Later on, Wadatsumi and the Sun Pirates are told by Jinbe to leave Totto Land during the ceremony due to security being more busy with the events than focusing on them.
- Wadatsumi vs. Surume
- Wadatsumi vs. Monkey D. Luffy
- Wadatsumi vs. Surume (Gyoncorde Plaza)
- Wadatsumi vs. Sanji and Jinbe
Translation and Dub Issues
"Onyudo" (大入道 Ōnyūdō?), Wadatsumi's nickname, literally means "large, bald-headed monster" or "giant"; the FUNimation subs use the term "Giant Monk". "Umibōzu", which is what Usopp incorrectly refers Wadatsumi as, means "sea bonze", a Japanese mythological spirit. Wadatsumi's name itself is derived from "Watatsumi", the name of the Japanese sea god and dragon of Shinto religion.
- He is the largest fishman to date and the only one so far to have no obvious fishman appearance to him, aside from a dorsal fin and webbed hands.
- Wadatsumi has his own unique laugh that goes "Fugufugufugu". Fugu is the Japanese word for blowfish.
- Similar to Fukuro, in the anime Wadatsumi is given a childish voice that does not match his physical appearance at all, but matches his childish personality.
- Wadatsumi is the only major character who ate an Energy Steroid without showing signs of age or weakness from the drug's side-effects(possibly due to only eating one at the time).
- Umibōzu - Wikipedia article about the legendary sea monster Usopp mistakes him for.
- Watatsumi - Wikipedia article about the Japanese sea god/dragon he is named after.
- Takifugu rubripes - Wikipedia article about the species of fish he is.