water-chestnuts, caltrop
薺 centers on aquatic plants with edible or useful parts: water-chestnuts and caltrop, both water plants with distinctive nut-like fruits.
薺 combines the grass radical 艹 with 齊, which likely contributes the sound. The character originally referred to specific water plants, possibly the water-chestnut or shepherd's purse, though the exact historical development is uncertain.
The grass top 艹 sits above 齊, like a cluster of water-chestnuts or caltrop fruits floating on the water's surface among reeds.
For セイ, imagine a calm lake where you say 'say' to the water-chestnuts as you pick them: say -> セイ, and the grass-like plants float on the water.