成唯識論述記 3.611

3.611

述曰：此第三喻。漂水下魚、水上之草，喻內習氣、外觸受等內外法也。

The Commentary states: This is the third metaphor. The fish swept beneath the water and the grass upon the water are metaphors for internal vāsanās and external internal-and-external phenomena such as contact, sensation, and the like.