Depending on the outside temperatures, white smoke may in fact be steam. Now, where the water came from is important, as if it is from the inside of the engine, it can be really bad. But, if it's external from a big puddle, washing it, etc., it's no big deal other than surprising.
Not sure when BMW eliminated the oil dip stick, but if you have one, look at the oil after you pull it out. If it looks chocolaty rather than the normal oil color, you have a coolant leak into the oil. If you have a head leak, that's bad, too.
Might be nothing, but definitely needs to be checked before big damage could occur.