Fish-man Kenjutsu

Fish-Man Kenjutsu is a martial art that is practiced by certain skilled members of the Fish-man race, alongside Fish-Man Jujutsu. It is a style that mainly revolves around sword and striking techniques. While deadly enough on land, Fish-Man Kenjutsu has a number of devastating underwater moves designed to suit the enhanced strength and speed that fish-men possess while in their natural environment.

The secret of Fish-Man Kenjutsu is mastery of the water in the user's direct vicinity. The sea itself is a Fish-Man Kenjutsu user's most effective and powerful weapon, and in addition to actual water, users of Fish-Man Kenjutsu can utilize the moisture in the air and even the water inside of people's bodies. It is also possible for a user to put their own spin on the style, so as to take full advantage of the characteristics of the fish species whose attributes they were born with, among other physical features that they possess.

While stronger in the water, Fish-Man Kenjutsu can still be deadly on land, as one can use it to make the moisture in the air carry the force of their strikes to their target, meaning the user can still land a blow on their enemy without needing to make physical contact. Additionally, because every creature has a high percentage of water in their body, the users of Fish-Man Kenjutsu are able to drastically increase the damage caused by their blows via the force of the impact traveling through the water within the opponent's body. This enables them to battle opponents that are usually highly resistant or immune to blunt force blows. 

Gedan-no-kamae

Starting at 3rd level, you have trained extensively to master your blade, allowing you to perform deadly attacks with the essence of the sea.

Leviathan's Aegis. At 3rd level, you can deflect both ranged and melee attacks. When you are hit by a melee attack, you can use your reaction to flow with the strike, reducing the damage by 1d10 + your dexterity modifier + your monk level.

If you reduce the damage to 0, you can choose to spend a ki point to redirect the attack at another creature that is also within striking range of the creature who attacked you as long as the attack also would have beaten that creature's AC. The redirected attack deals the damage it would have dealt to you plus your dexterity modifier.

Shigure Soen Ryu

At 6th level, you have trained your mind and technique and it allows you to better use the power of the sea from around you and within you. You are able to utilize the following features:

As you draw your blade to strike, you are able to use you Wisdom modifier to your initiative rolls. Also, the first time you hit a creature with a melee weapon attack during the first round of combat, you can spend 1 ki point to deal additional damage equal to two martial arts dice instead of one.

Swallow's Strike. Attacks that you make with weapons are imbued with water and are considered magical. Additionally, you are able to launch water waves with your sword from afar and they bear the sound of a swallow's scream. You may use your swords to make both melee and ranged weapon attacks. Ranged attacks have a range of 45/120 feet. This attack uses the same damage die as the weapon's melee attack, but always deal slashing damage. Also, if hit by these attacks, an additional 1d4 of cold damage is dealt.

Steel Vortex

At 11th level, you can use your action, and spend 2 ki points, to make a melee weapon attack against a number of creatures within 5 feet of you; separate attack rolls for each target. 

Poseidon's Aura

Poseidon's Aura. At 17th level you can use 5 ki points to have the aura of the Sea King Poseidon making you nearly untouchable. This state lasts for 1 minute or until you make an attack or cast a spell. While you are in this state, you automatically succeed on any dexterity saving throws and any attack rolls made against you have disadvantage on them.

Sea King's Punch. While in the state, you can absorb the momentum of attacks made against you and unleash it all at once in one devastating blow. If you make an unarmed strike against a creature while still channeling this state, the strike deals an additional 1d10 force damage, plus an additional 1d10 for each attack you avoided while in the state, up to a maximum of 10d10 additional damage.