This is all wishy washy without an actual court, but I’m dubious this would rise to that standard.
From what I can tell, international law requires attacks to be distinct and proportional. Distinct meaning they distinguish between civilian and military targets (you cannot intentionally attack civilian targets), and proportional meaning that the collateral damage to civilians is proportional to the military value of the strike.
This probably doesn’t meet that bar. Cutting off water is bad, but 20k people is a fairly small population for this kind of thing and presumably the US won’t stop them from repairing it (that probably would be a clear cut war crime). Presuming Irans government is functioning, this should be extremely hard for those people but probably not lethal. Trucking water around is an option, even if it’s not great.
My problem is that this happened after Trump repeatedly and emphatically threatened to target civilian infrastructure. It could be an accident, or it could be exactly what he said he would do.