I grew up in central California and from a young age (as in I cannot even remember not being told this) we were warned not to play in the canals. farm country that had huge concrete canals running all through the town that the farmers would have the resevor opened up in case of drought/dry times and have the water brought to the farms, it would rush down the dry canal beds REALLY INSANELY FAST and if you were in the beds, wash you out with it! They were very very tall with almost straight sides. They were fenced in etc and no one lived on them or backed onto them (city ordinance) but still as a child almost every time we drove by them we were told to never go in them etc. I think someone literally would have had to shove me in to make me go in. We had discussions in school about what if someone was in one and needed to get out. (tell them to wait and not move, go run to the nearest store/etc and explain, get a grownup.)
I think the practical side of it needs to be discussed, when it is anything dangerous, we live in a very busy suburb and dd has known about the road since very young. When they are little they should be told "no" firmly/sternly and led away and saying somethign like "we cannot be near there it is danger!" in a very stern voice. Children understand firmness, and tone of voice very young. Then as they are older and able to talk you can say why its dangerous etc, Just always step up the teaching of why its dangerous and how to be safe around the dangerous area. I am sure some people won't like the use of the word danger but I think its necessary they need to understand that this is serious and that you mean business.