Water is not a fire retardant. Water can extinguish fire, but you can't apply water on a forest to prevent a fire from spreading there in the first place.
Your last paragraph seems to agree with parent? We should know what's inside, but it might still be the best solution.
More precisely, not nearly as effective. The fire retardant is effective hours or days after being applied. Water would have long since evaporated and had almost no effect. Even on very short timescales, the retardant is still much more effective than water alone.
Your last paragraph seems to agree with parent? We should know what's inside, but it might still be the best solution.