Water does not deliberately disobey a moral command, so the ordinary human meaning of “transgression” cannot be predicated of water without figurative adjustment. The mention of water helps indicate the nonliteral sense.
The likeness is exceeding a limit, but the nature of excess differs. Human transgression may be intentional and moral; the flood's excess concerns quantity, level, and spread.
This shows that a metaphor may borrow from a conceptual meaning and apply it to a sensory event. “Faṣdaʿ” illustrates the reverse direction, from sensory action to conceptual proclamation.