deep and broad (water)
滉 centers on water that is deep and broad, suggesting a vast, expansive body of water.
滉 combines 氵 (water) with 晃, which likely contributes both sound and the idea of brightness or vastness, together suggesting deep, broad water.
The water radical 氵 beside 晃 (bright, vast) paints a picture of sunlight sparkling over a deep, wide expanse of water.
For コウ, imagine a vast ocean cove where the water is deep and broad: cove -> コウ, and the cove's water stretches endlessly.
wide and deep (of a body of water)