The fountain speaks to you.
"The power of your own passion can save your kin and bring others to the sea. It means I will give through this fountain a fragment of its essence into you, that you may pass it on to the mers, to finish the incomplete mermen and free the minds of the mermaids. This power will allow you to gift humans, to make mermaids or mermen from humans of either gender."
"What?" You ask. "Wait...I could turn other boys into mermaids?"
"If they so desire. Or girls into mermen."
"How?"
"To mate with your opposite will produce the change and preserve the gender. If you mate with a male, you will have a merman. A merman mating with a female will create a mermaid. But if you, a mermaid, allow a male to eat at your womanhood, he will become a mermaid. Conversely, if you allow a woman to suckle at your breast, a merman will be born."
You blink. "How does it work the other way around? I mean what do I teach the boys?"
"A merman can make a mermaid of a male if he takes the male anally. If a female should suck at his member, it will make her a merman."
"Hmmm..." You wonder, thinking of a plan. "What of the mermen already here?"
"Did you not see?" The Gaia fountain says. "They are mostly incomplete. They will be affected as human men would."
A plan begins to form in your mind.