dike, embankment
圩 centers on a raised barrier of earth that holds back water: a dike or embankment.
圩 combines 土 (earth) with 于, which likely contributes the sound. The character is used for earthen dikes or embankments.
The left side 土 is earth, and the right side 于 looks like a barrier with a line through it. Picture a wall of packed earth holding back water: a dike.
For ウ, imagine a dike holding back water with a low 'oo' sound as the water presses against it: oo -> ウ.