clear and deep (of water), wide
泂 centers on a quality of water: being clear and deep, or by extension, wide and expansive like a vast, transparent body of water.
泂 combines the water radical 氵 with the phonetic component 冋, which likely contributed the sound. The exact historical development is uncertain, but the character is associated with the idea of clear, deep water.
The water radical 氵 beside 冋 suggests a deep, clear pool. Imagine looking into a wide, transparent lake where you can see all the way to the bottom.
For ケイ, picture a clear, deep lake shaped like the letter K: K -> ケイ, and the water is so clear you can see the K-shaped bottom.