That really depends on the particular policeman you find.
Once I was having lunch with some friends in a park and one of us was drinking a beer can. We were alone there and, certainly, not causing any problem. The police approached us and fined my friend for drinking in public (I don't remember how much it was, this was some years ago). I live in front of a park now and I've seen similar things. I've also been drinking many times in public without any problem, and I am quite sure the police noticed us.
The Netherlands is, in general, a very tolerant country, but not as much as some people think. Maybe it is compared with America (although, in my very limited experience, I've never had any problems with the American police), but the difference with most European countries is quite moot.