As a fishman you can breathe under water. You also can move extremely fast. You can breath above  water as-well.

As a fishman you have a plus 2 to strength and you gain advantage on all attacks made in water

You have a powerful bite attack that deals piercing damage equal to 2d6 + your Strength modifier. When you hit with this bite, you can choose to extract a piece of your target. If successful, you regain hit points equal to half the damage dealt by the bite.

Fish man karate is a form of martial arts only known by fishmen. It uses water as a weapon. 

Bioluminescent Camouflage: You can emit a gentle bioluminescent glow at will, allowing you to blend seamlessly into the darkness of the ocean depths. This provides you with advantage on Dexterity (Stealth) checks while submerged in dim light

Deep Sea Adaptation: You have darkvision with a range of 60 feet, and you are resistant to cold damage. Additionally, you can endure extreme underwater pressure, allowing you to dive to depths that would be harmful to other creatures without suffering any ill effects.