I decided to make some rules for my lists:

1. I’m only going to include anime characters from anime/manga I’ve read or watched. Mainly because I would be familiar with their powers.
2. I’m only going to include ONE character per series. This is difficult to do, because there are many strong characters in any given series, but I want some variety.
3. I think it will be difficult to determine how each character on the list would fare against each other, so I going to focus how strong they are within their series. However, I will be thinking how they compare relative to each other.

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[Note] This list and any others will focus on characters that MOSTLY or significantly use the element. In this case water. What that would result in is having a list of characters that will have more that just the element in their abilities and I will be factoring these other abilities in as well.

5. Tier Harribel (Bleach)

Harribel is the former 3rd Espada in Aizen’s army. She eventually becomes the ruler of Hueco Mundo. Like most Espada, Harribel has tremendous skill with a sword. She’s capable of holding her own against a captain-level Shinigami like Hitsugaya. She was even able to hold off a captain and two lieutenant-level Visored together.

She possessed incredible spiritual power and was highly perceptive during combat. Tier was considered to be a master tactician. Her hierro “harden skin” was strong enough to survive Hitsugaya’s attacks without much damage. She also has abilities typical to most Espada like Cero, Sonido and Descorrer.

When Harribel uses her Resurreccion, she releases her special ability. With her new abilities she is able to manipulate water. Her best water techniques are:

• Hirviendo – Pointing her blade out, Harribel boils any form of water which comes close to it, including ice, and deflects it around her.
• La Gota – Harribel can condense large amounts of water around her blade before firing it as a blast which resembles a shark tooth. She can fire multiple blasts in rapid succession with relative ease. The blasts strike with enough force to demolish houses.
• Cascada – Harribel launches a rushing surge of water at her enemy. The torrent of water is large enough to cover several town blocks.

Source: Bleach Wikia

4. Juvia Lockser (Fairy Tail)

Juvia is a S-Class Mage from the Fairy Tail Guild. She can produce, control and manipulate water. She has a variety of different ways she can produce water, for example she can produce water blades that are strong enough to slice through rock or metal. She can also turn herself into water, allowing physical and magical attack pass through her without damaging her. Some of her best techniques are:

• Water Lock – Juvia has the ability to materialize a large circular mass of water, and unlike the Water Dome, it contains no oxygen inside. This spell is used to capture enemies and incapacitate them by suffocation.
• Sierra – A full body transformation resembling Water Body, used by Juvia in a state of strong agitation or other influencing emotion (e.g. jealousy or rage). Her strong emotions cause her full body transformation to heat up/boil. This is a short ranged attack which displays enormous, unpredictable speed.
• Water-Make – a form of Molding Magic that lets Juvia create, or at the very least manipulate existing water. The scope of usage is not limited to just pure water, but also to things that contain it in large quantities, such as blood.

Source: Fairy-tail wiki

3. Giyu Tomioka (Water Hashira)

If my understanding is correct, technically the breathing styles only mimic the elements. So water breathing is “like” flowing water no actually water. I’m going to count it anyway.

Giyu Tomioka is the Water Hashira from the Demon Slayer series. Tomioka is a stoic character with a strong sense of right and wrong. As a Hashira, Tomioka possess speed, reflexes, stamina, endurance, and strength. His breathing style, Water Breathing affords him a number of techniques. Giyu is the most skilled Water Breathing in generations completely mastering the style. He’s even created new skills in the style the Eleventh Form: Dead Calm.

Tomioka is so strong he’s able to fight against Upper Rank Three, Akaza, who claims that “he has not fought a Water Hashira as skilled as Giyu in fifty years.” Against Muzan, Giyu was able to stall him long enough for Tanjiro to arrive.

Tomioka’s Strongest Techniques:

Demon Slayer Mark – when awakened Giyu’s speed, precision, and endurance all increases.

• Ninth Form: Splashing Water Flow – The user changes their footwork in a way that minimizes the landing time and surface needed when landing, allowing the user to move without limits. Ideal when fighting in a place with no solid foothold.

• Tenth Form: Constant Flux – The user performs a continuous flowing attack which seemingly takes the form of a water dragon that increases in strength and power with each rotation due to momentum.

Source: Kimetsu no Yaiba Wiki

2. Tobirama Senju (Naruto)

Tobirama was considered one of the most powerful shinobi ever. He was considered the “fastest shinobi,” at least during his time. His chakra control was such that he could use difficult techniques with just one hand seal. Like most ninja, Tobirama had a large number of sill sets, from summoning techniques, barrier ninjutsu to the Shadow Clone Technique.

Tobirama was an “adept sensor.” He could “accurately detect the presence and location of targets in the surrounding area. He could even sense persons countries away, identify a target’s ancestry from their chakra signature, and even pinpoint a target’s exact location while blinded,” simply by touching his finger to the ground.

While, he could use all nature transformations he mastered Water Release. He could create large amounts of water without a water source. He could use high level water jutsu at any time. He could shoot high pressure water beams that could slice through his targets, create walls of water and cover an entire battlefield with water. Some of his best techniques were:

• Flying Thunder God Technique – he could instantly teleport himself and/or other targets to any location marked with his seal. 
• Summoning: Impure World Reincarnation – which revives the dead into immortal, regenerating bodies.
• Mutually Multiplying Explosive Tags – to take advantage of the revived bodies’ natural advantages, using them in kamikaze-like attacks to create near-infinite chains of explosions on a target. He was said to use it whenever he needed to clear a map.

Source: Naruto Wikia

1. Jinbe (One Piece)

Jinbe was the second captain of the Sun Pirates. A former Shichibukai he possessed massive strength. He is so far the strongest fishman seen in the One Piece series. Despite his large size he move quite fast as well. Jinbe also has impressive endurance, able to take a “baptizing” in boiling water.

He was able to hold his own against Big Mom, he even took a direct sword attack from her. While in water he becomes even more powerful, he an excellent swimmer and he’s able to “suppress” any fear he may have. He has mastered Fish-Man Karate, he’s a great strategists, he can communicate with fish life and has become skilled in Haki. His most powerful abilities are:

• Fish-Man Karate – According to Jinbe, the secret of Fish-Man Karate is mastery of the water in the user’s direct vicinity, using its power to send powerful waves impacting the water within the opponent’s body. Because every creature has a high percentage of water in their body, the users of Fish-Man Karate are able to battle opponents that are usually highly resistant (if not immune) against blows and punches. 
• Busoshoku Haki – is a form of Haki that allows the user to use their spirit to create, in essence, an invisible armor around themselves.
• Buraikan – A secret technique of the Fish-Man Karate. Jinbe concentrates in one hand a large amount of water that he shoots at close range against his opponent. The impact is so strong that the water penetrates the opponent’s body and gets out from the other end. 
• Gyojin Jiujitsu: Water Heart – Jinbe plunges into the sea. Using a jujitsu technique (unarmed martial art that originates in the bushi battle methods), Jinbe centralizes in both hands an enormous amount of water, to then transfer it to the surface. 

Source: One Piece Wiki

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So do you agree? Is someone too high or too low? Think I should do more lists? What kind. Let me know what you think.