Dark Life by Kat Falls
See into the future. It's finally happened, just like the doomsday guys have predicted - great chunks of land have slid into the ocean. The topsiders are crammed into skyscrapers, while a brave few live under the sea and grow the food that keeps humankind alive. Ty is a teenage boy who was born and raised on the sea floor, and he's developed some unique abilities, which he keeps secret at all costs. While exploring a deserted submarine, he finds a girl named Gemma who ran away from the topside to find her brother.
Suffice it to say that there's action, mysteries, daring does, villains, strange abilities, crooked politicians, secret prisons, and budding relationship - this book pretty much had it all. For more adventures with Ty and Gemma, pick up the sequel - Riptide