Rivethanael

Known as River Elves, the Rivethanael are a blue-green-haired species of Elf is actually amphibious. They can shift between freshwater-breathing and air-breathing forms in a process that takes five minutes of partial inundation. This causes their feet to transform into fins and spines on their arms to swell into thicker fins, and their gills, shut tight in air-breathing form, to open. While in their underwater form, they can swim deftly but will suffocate if too rapidly removed from the water and can injure themselves if they try to walk. Additionally they will injure themselves (or their clothes) if they are wearing boots or sleeves during this transformation. In either form they cannot survive in salt water.