I've thought about this issue before myself, and I came to the conclusion quite a while ago that a barrier of some sort isn't needed.
First of all, if a small child did fall in the lake, then in my opinion it is the parents fault for not supervising their child properly. Any child over the age of about, say, 6 or 7 should have the sense to work out that they shouldn't mess around near the edge.
Secondly, a drunk is taking the risk by getting drunk, so if they fall in it's their fault really.
Finally, all those lakes that you see in public parks don't have barriers, so why should this? And how many times do you hear of people falling in? Rarely. As far as I know, nobody has had an acident at Lake Disney yet, and if someone did they should be able to easily climb out since it's so shallow.