rice paddy ridge, furrow, rib
畦 centers on a raised dividing line in a field: a ridge between rice paddies, a furrow, or any rib-like raised strip that separates sections.
畦 combines 田 (rice field) with 圭, which likely contributes the sound and the idea of a raised earthen boundary. The character expresses the ridges that divide rice paddies.
The left side 田 is a rice field, and the right side 圭 looks like piled earth. Picture the raised earthen ridges that separate flooded rice paddies.
For ケイ, imagine a kay (ケイ) floating along the ridge between rice paddies: kay -> ケイ, and the boat follows the furrow.