You were a mermaid. You lived peacefully in water, swam around, minded your own business, told hunters you weren't a threat and to please leave you be. Occasionally, you would go on land to get human food, which was very popular with mermaids. But you'd begun noticing a change in the food. And obviously, you unaware of the leviathan situation happening topside.

So, that's how you ran into Sam and got some info on the whole leviathan thing. You then opted to go with Sam back to his motel room, where you met Dean, which meant explaining your situation again, which meant-

"I'm sorry, you're a what?" Dean asked, as if making sure he heard you correctly. Mermaids weren't real, is what he thought. Then again, a lot of things he thought weren't real ended up being very real, so, who was he to talk?