You generally use distilled water in the cooling system (you could get from an automotive store like halfords), although you can use soft water ie: untreated water, I have used this in race bikes (aluminium engines and radiators) for years with no problems, the problems arise when using hard water (town supply water).
For most aluminium radiator bikes you use a mix of 50% water and 50% anti freeze (liquid cooled). any stronger mix of anti freeze and your bike will not warm up properly. Your BMW manual should also tell you the correct mix, or you could email BMW and they will tell you.
In South America and heading to Tierra Del Fuego ETA Feb 06
Finished 09/04/06, Denver Colorado to Ushuaia and back to Santiago. 27000 miles two up on a 2005 F650GS Dakar