I agree with yi en, make it a trigger.
Too many maps already use low water purposely for things like ladders, riverbeds, etc.
Having this as a default function would ruin it.
Also, making it a trigger would allow it to be flexible. Like attaching a timer to it.
1 minute the players are running on the low water, the next they all fall and have to swim in it. Just a thought.
Not bad, I like the idea!