The Man Fish Invasion of Florida ca. 1943 was undoubtedly organized by the Axis powers with German U-boats delivering the creatures to Florida and British possessions in the Caribbean. In the wake of the panic and destruction the War Department began research on several of the creatures that were captured or killed. They discovered that they were very similar to human beings and developed a series of treatments to give operatives the ability to breathe underwater and increase their physical power. These operatives were to become a unit of seagoing commandoes -The Undersea Task Force.
The high point of the Undersea Task Force came in the winter/spring of 1944. Working jointly with a British invisible man, several American mystery men, and a trained dolphin, they uncovered an attempt by the Italians to destroy the English Fortress of Gibraltar using a giant underwater robot, a suicide sub and a number of blackmailed Atlanteans*. Several aquatic men lost their lives and their dolphin companion gave his life to carry an explosive into the robot's workings but the plan was foiled. Subsequently contact with Atlantis brought the water breathers into the Allied forces.
*Coming soon or stat your own!
The Aquatic superpower has three levels.
Low Powered Aquatic- The character can breathe water (salt of fresh) indefinitely. They also gain 4 armor points. The character is immune to any temperatures short of boiling water or arctic conditions and ignores pressure changes. They can scare predators like sharks away with a Charisma save. The character must return to water frequently to hydrate. To represent this they have a 1d6 usage die. Each fight on dry land or exposure to fire attacks requires a die roll. Failure to return to the water forces them to make all rolls with disadvantage.
Paranormal Powered Aquatic- As above but the character's usage die is 1d8. The character receives an additional 2 armor points. He can make leaps out of the water to attack nearby targets, Their eyes adapt allowing them to see in all but total darkness. Boiling water or arctic conditions are harmless to them.
Superhuman Powered Aquatic- As above but the character's usage die is 1d10. The character receives an additional 2 armor points (eight total). The character attacks underwater targets with advantage to hit and damage and likewise saves with advantage vs. all attacks. They can coerce sea life to obey simple instructions with a Charisma save. This can give them advantage to some rolls using the creatures as a diversion or attacks letting them roll their damage with advantage.