My suggestion about posting the rules wasn't intended to include detailed information on how to properly cave dive -- clearly that would be way too complicated. Also, I didn't intend that the wording in my post be the exact wording for the sign. For example, I would probably say something more like "Do NOT attempt to enter this or any underwater cave without using guidelines specifically designed for this purpose, and being trained in the use of the appropriate guidelines" and "Do NOT attempt to enter this or any underwater cave without being trained in and understanding the Rule of Thirds" and so forth.
My idea, and it is just a preliminary suggestion, was that by giving a few more specifics, the sign might be more effective in deterring untrained divers, because it may help some of them realize that they need to be aware of more considerations than in recreational diving. If a diver reads the sign and wonders "what IS the rule of thirds, anyway?" or "what do guidelines specifically designed for this purose have that my rope doesn't have?" then at least they will have recognized that there are some more things involved in cave diving than what they had already thought about.
Obviously, there are some that will still ignore the sign, telling themselves that they DO know enough about guidelines, the rule of thirds, and the other rules, even though they are not trained. But those divers are likely to ignore any of the warnings discussed so far.
Another suggestion: Include the statistics on the sign? Things like the fact that only 4 out of 480 missing cave divers were rescued alive, and the percentage of cave-diving deaths that were associated with breaking any of "the rules" should certainly give some rec divers enough reason to reconsider before they go into the cave without following the rules.