I was also surprised that my E9x didn't have a water temp gauge.
I learned the hard way recently that the E9x DME will store "hidden" codes related to things like water temp / water pump function. So if your water pump starts to fail it will know long before you get stranded.
It's a good idea to ask your shop to check for those BMW-specific fault codes every once in a while. Otherwise don't worry about it until the dreaded light comes on.