furrow in field, small drainage
圳 centers on a man-made water channel in farmland: a furrow or small drainage ditch used to direct water through fields.
圳 combines 土 (earth, soil) with 川 (river, stream), suggesting a water channel dug into the earth. The exact historical development is uncertain, but the component combination fits the meaning of a field furrow or drainage ditch.
The left side is 土 (earth), and the right side is 川 (river). Picture a small river carved through the earth of a field: that's a furrow or drainage ditch.
For ショウ, imagine a farmer showing you the furrow in the field: show -> ショウ, and the earth channel guides the water.