offing, open sea, rise high into sky
冲 centers on the idea of rising or surging upward, like water rising into the open sea or something soaring high into the sky.
冲 is a variant form of 沖, combining 冫 (ice) with 中 (middle). The exact historical development is uncertain, but it likely relates to the idea of water or cold surging upward.
The left side 冫 suggests ice or cold, and the right side 中 suggests the middle. Imagine a geyser of icy water shooting up from the middle of the open sea, rising high into the sky.
For チュウ, picture a huge column of water shooting up in the middle of the sea: the sound of the rushing water is like a long 'chuuu' — チュウ.