As long as you're not in super sub zero temps it will probably be ok but your coolant will be more diluted than it should be - you could add some concentrated super coolant to the mix but I'm not sure how many gallons of plain water would remain in the system to calculate how much concentrate to use.
when did you buy it and is it still under 3/36 b to b warranty or the 5 60K power train warranty if so, back to Subaru and tell them to pressure test it hot and cold and burp the systemI bought a 2017 Outback and the coolant is pretty low in the reserve tank. I added a couple bottles of fresh drinking water to the tank to fill it up.
Now I'm not sure what to do to get the right mix of coolant in the engine. I don't know what "type" of coolant the previous owner used. When I look in the tank and the radiator it just looks like water. Not blue or green or anything.
Should I drain or syphon out the some water and add Subaru or super blue coolant?