Cute! I'm not sure who the target of that sign is, or who would be liable in a lawsuit if someone was surprised by the natural properties of water. But I once had a dog who was.
I was camping at Garner State Park in the Texas Hill Country, and had an 18-month-old dog along. In retrospect, I guess he had never experienced deep water, knowing just rain puddles, and apparently his doggie instincts didn't prepare him for anything else.
My camper was near the bank of the slow-flowing Frio River, and one morning at dawn the water was absolutely still and mirror-like. A pair of ducks were about 10 yards out, moving so slowly they made virtually no ripple on the surface.
My dog saw them, got all excited and decided to chase them. He leapt off the raised river bank, splashed down several feet out, and went completely under water. He surfaced all panicked and thrashing wildly, and I got into the water to rescue him. The shocked look in his eyes was both funny and pitiful to me.
He evidently thought the still water was no different than a rain puddle that he could run across. Whether that Australian sign is for humans who might make the same mistake I don't know. And if there are any dogs smart enough to read signs, I doubt even they'd be too stupid not to already know what deep water is.