That's gotta be warning that the bottom drops off quick

Warning: dude to your left has his dick out.

Caution: Mermaids with strapons, waiting on subsurface rocks.

"Free Handjobs" But that's how the sirens getcha.

Common in Florida, this sign warns tourists that locals may be wearing a banana hammock.

Apparently a real Australian safety sign: https://www.royallifesaving.com.au/stay-safe-active/communities/multicultural-communities/how-to-stay-safe (scroll down to the "Out and about" section)

Warning: If you wear a thong instead of swim shorts, your homies will get turned on when you tread water in front of them.

It means sex on the beach is allowed, but only if one of you is about to drown. Everyone knows this.

"Must be 6 inches or shorter to swim."

That's clearly a siren or some kind of merfolk on the left, I think the sign is telling you this is their territory.

You may encounter nude bathers beyond this sign.

Watch out for hard dudes in boots

So now I'm thinking its some kind of instruction of how to stand and properly penetrate the other person