In a pinch, tap water is fine, as long as you flush it out later.
Distilled H2O is better.
The #1 reason people get rid of this car is cooling issue. Make sure you don't have a blown head gasket. Do NOT drive the car now.
What year/model/trans/mileage?
If you have < 120K miles, do a Partial Cooling Ovehaul.
If > 160K, do a complete cooling overhaul.
Info for partial overhaul is below:
DIY: 2006 BMW X5 3.0i PARTIAL Cooling Overhaul @ 116K miles
https://xoutpost.com/bmw-sav-forums/...16k-miles.html