(water) divining equipment
筮 centers on divination using bamboo or yarrow stalks, specifically the equipment used for water-related fortune-telling. The meaning is narrow and technical, referring to the tools of the practice rather than the act of divination itself.
筮 combines 竹 (bamboo) with 巫 (shaman, diviner), suggesting bamboo stalks used by a diviner. The historical development is consistent with this component analysis, though the exact ancient form is uncertain.
Bamboo 竹 over a shaman 巫: picture a diviner using bamboo sticks to tell fortunes near water, the equipment itself becoming the kanji 筮.
For ゼイ, imagine the diviner saying 'Zay, the water spirits speak!' as they cast the bamboo stalks: Zay -> ゼイ, tying the sound to the divining equipment.