deep clear water
泓 centers on deep, clear water: a still, transparent pool or body of water that is both profound and pure.
泓 combines 氵 (water) with 弘, which likely contributes the sound and the idea of vastness or depth, together suggesting a deep, expansive body of water.
The water radical 氵 beside 弘, which suggests something broad or expansive, creates an image of a wide, deep pool of clear water.
For オウ, imagine an old, deep well (オウ sounds like 'O' in 'old') filled with clear water: 'O' -> オウ, and the water is deep and clear.