I'd recommend buying a real deal street sign, and the exact same type of post that the street signs in your area are bolted to.
Our local ramp has a sign posted with a notice of all the gear that boaters and jetskiers need, along with the jacket requirement. And people do actually read it. Putting up a sign that says "this means divers below, stay 125' away" right next to that, would be effective.
And then you have a professional looking sign that appears to belong there, and might have been placed there by the state. Might not be much more $$$ than flyers or plastic cards.