shore, river in Hubei
澨 centers on a water's edge: a shore or bank where land meets water. The river name is a specific geographic application of that shore idea.
澨 combines 氵 (water) with 筮, which likely contributes the sound. The exact historical development of 筮 is uncertain, but the overall structure points to a water-related place name or shore.
The left side is water 氵, and the right side 筮 looks like a complex structure with bamboo 竹 and other elements. Picture a bamboo grove growing right at the water's edge, marking the shore.
For セイ, imagine saying 'say' as you point to the shore: 'Say, that's the shore!' The word 'say' cues セイ while you look at the water's edge.