Stir water, soy sauce, cilantro, and rice vinegar together in a bowl.
Heat olive oil in a deep skillet over medium-high heat. Saute ginger in hot oil until fragrant, about 1 minute; add the water mixture and flounder to the skillet. Bring the liquid to a boil and immediately remove skillet from heat. Place a tight-fitting lid on the skillet and poach flounder in hot liquid until the flesh easily flakes with a fork, about 10 minutes. Top fillets with chopped scallions.
