What if Cressa-la hadn't touched the water? How drastically different would things be?
Every baby is killed beneath the waves--with one exception. The one-hundred thousandth lives.
A long time ago, 1 in every 100,000 children born from a human was a mermaid. But now mermaids are the only ones that can survive in this flooded world. Every human child born is killed the moment they take their first breath. However, one in every one-hundred thousand is spared.
These children are placed on the mountain-tops, the "islands", that poke up over the sea. The Earth has become warmer, making these islands habitable. The humans that survive through Mother Nature's fury have created Tribes, created rules. One of these rules is to not go near the water, to not touch it.
And Cressa-la doesn't. Because she chose to follow the rules, could there be an even bigger consequence than when she had?