I personally think the sign as shown on page 1 of this thread is as good as it gets. The condensed version is so short, I don't see the point beyond pointing out newbies, and then it becomes a "newbie-centric" sign. Unsafe rider behavior isn't defined on the condensed sign.
On the other sign you focussed in on jumping close and riding close to shore.
Is it that we don't want to list those ideas on a sign?

I only know me. If I pulled up to Jupiter or St. Augustine... etc. and I saw a sign, I would read it for sure. I would actually feel better seeing a sign and being able to read about the local expectations. Obviously, if most riders were breaking most of the rules on the sign, I would probably just do what I wanted. If they were following them though, so would I.
That's just me though. Some won't care about the local "rules" and disregard a sign.