Is this the first time it's ever needed topping up?
I would suspect that if it is, then it is high time for a coolant change. Most coolants have a corrosion package good for 5 years and some for 10. We can't be sure which your car has, so I would suggest a change and flush. At 10 years old it would be due anyway, whichever coolant is fitted. Have the system checked or leaks at the same time.
In the meantime, if this is the first time its needed a top up, then topping up with water will do no harm. If you continually top up with water then the coolant concentration will be diluted, possibly to the point where corrosion becomes a real problem.
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