You were a siren of the depths, and you were on the surface to feed. It was easy to attract humans to the sea just by opening your mouth and singing at the frequency of angels. Beaches weren't Dean's thing, that's for sure. He stood in the hot sun, watching. There were reports of children and men disappearing at sea, or on the fishing coast.
There was a difference between sirens and mermaids, sirens were more dangerous for sure. Dean packed everything, harpoons, and other weapons and waited, putting some blood in the water to attract. "Come on, show up, bitch fish."