ear (of grain), head, crest (of wave)
穗 centers on the tip of a plant or a wave: the ear of grain, the head of a stalk, or the crest of a wave. All meanings point to the topmost, often heavy, part of something that grows or rises.
穗 combines 禾 (grain plant) with 惠, which likely contributes the sound and may carry a sense of something hanging or drooping, like an ear of grain. The exact historical development of 惠 within this character is uncertain.
The left side 禾 is a grain plant, and the right side 惠 looks like a heavy cluster hanging from the top. Picture a stalk of wheat bending under the weight of its ripe ear at the crest of the plant.
For スイ, imagine a swan gliding over a wave crest: swan -> スイ, and the swan's neck curves like the crest of a wave.