an aquatic grass, the stalks of which are eaten as a vegetable
茭 refers to a specific aquatic grass whose stalks are eaten as a vegetable. Its meaning is narrow and botanical, without broad abstract extensions.
茭 combines the grass radical 艹 with 交, which likely provides the sound. The character was created to name a specific plant, but the exact historical development is uncertain.
The grass radical 艹 sits above 交, like crossed stalks of an aquatic plant. Picture the crossed stems of this edible grass growing in water.
For ケキ, imagine a chef making a cake with the aquatic grass stalks: cake -> ケキ, and the crossed stalks 交 under the grass 艹 become the special ingredient.