If you were living in frigid to below freesing temps then the high water content would be a problem. Real concern is ability to prevent freezing when using coolant. So if your 70/30 is calibrated to your climate, things will stay cool.

The second and very important factor is the anti-corrosion properties of the coolant. That is one reason why 50/50 is so widely recommended and used
If you use distilled water and MB coolant with coolant change within every 36 months, that is good maintenance.