It depends. A mixture of more than 50% water in antifreeze will lower the boiling point in the summer and raise the freezing point in winter. Here is some information from a web site about testing the engine coolant.

(Test the antifreeze in the fall and in the spring. Antifreeze protects against the coolant freezing when cold and boiling over when hot. Most car manufacturers require a mixture of 50% distilled water to 50% antifreeze. Use an antifreeze hydrometer to test the freeze and boil levels. Antifreeze mixed at 50/50 will have a freeze protection to –34 degrees Fahrenheit and a boil protection up to 265 degrees Fahrenheit. When testing the antifreeze, do not open a hot radiator cap. Severe burns could result. If you have to add antifreeze check in your owner’s manual to make sure you are adding the correct type.)

Hope this is of some help.

 

Why not be safe and flush cooling system then fill with proper coolant?

 

Or if you know how much water was added, fill with an equal amount of non-premixed coolant. If you are talking about a pint of water being added; then I wouldn't worry about anything.