"Er, am I immortal now?" You ask.
"Of course" One of the mermaids answers. "We all are! We can do this forever!"
"Well, that's good news!" You answer. "And exactly what is "this" you speak of?"
The lead mermaid responds by swimming over to you and impaling her vagina on your tongue.
"Oh. That's what you mean." You say, your voice muffled by the flesh of her Venus mound. You close your eyes and let yourself be lost in the amazing feeling of cunnilingus with these mermaids, as another one swims over to you and shoves her tongue up your snatch. Then, you start to get curious about what being a mermaid actually means, and you pull out for a second to discuss it with them.
"Do I have any special powers of any sort?" You ask.
"Certainly!" The lead mermaid replies. "For instance, you can summon any manner of sea creatures from out of the blue, create and control water, and turn humans into mermaids!" You are stunned at this revelation, and are eager to try your new powers out, starting with the human transformation thing.
"But, to turn humans into mermaids, wouldn't I have to get out of the water?" You ask.
"Nonsense! You can control water, so you can create canals leading out of this pool!" The lead mermaid replies. "We just haven't done that yet, because we don't want to reveal our existence. We have to be cautious when trying to perfect the world."
You think about this for a second, then you realize that your knowledge of human settlements will be helpful in converting the rest of humanity. "If you really want to change the rest of the world, then follow me!" You say. With that, you create a small rivulet from the edge of the pool, and swim down its length. The mermaids follow behind you.
"Where are we going?" The lead mermaid asks.
"To my house. I want my sister to see this!" You reply.