Because sharks are known for tearing people to shreds, even not too far off the coast of beaches.
Also, humans are easy prey for a shark in water compared to say a crocodile on land, at least you could possibly run away or go indoors or something. In water vs a shark you are pretty much dead.
And I'm talking more about Great Whites here. I've heard far too many stories of people getting eaten by them on beaches here to think they aren't that bad.