at the mercy of (the waves)
隨 centers on being carried along by external forces without resistance: being at the mercy of something, following passively, or yielding to circumstances.
隨 combines ⻖ (mound, hill) with 遀, which likely contributes the sound and the idea of following or yielding. The exact historical development of 遀 is uncertain.
The left side ⻖ suggests a slope or hill, and the right side 遀 looks like something flowing downward. Picture a leaf being carried down a hillside stream, completely at the mercy of the water.
For ズイ, imagine a leaf drifting at the mercy of a lazy river, making a soft 'zooey' sound as it swirls: zooey -> ズイ.