water plant, nymphoides peltalum
荇 centers on a specific aquatic plant, Nymphoides peltatum, a floating water plant with yellow flowers. The broader sense is simply a water plant.
荇 combines the grass radical 艹 with 行, which likely provides the sound. The character is used for a specific water plant.
The grass top 艹 over 行 (going) suggests a plant that goes on water: a floating water plant like Nymphoides peltatum.
For コウ, imagine a koi fish swimming under the floating leaves of the water plant: koi -> コウ, and the plant drifts above.