Seamark by Cari Z.

A distant island in the blustery north holds a secret – a clan of shapeshifting sea dragons called the Agnarra, all but wiped out by humans a generation ago. For Morgan, a lone artistic soul among his practical people, life on the island is incredibly boring. That is, until a human ship explodes on the horizon, causing fear among the clan.
In the aftermath, Morgan finds a human in the water – Auban – badly injured and with no memory of his past. As Morgan nurses him back to health, he can’t help but fall in love. But their love is forbidden – Morgan’s people, especially his fierce older brother, would kill Auban if they knew he existed. It’s up to Morgan to keep him a secret until they can find a way for him to escape the island.
But secrets never last, and neither does the Agnarra’s new peace. War is coming, and Morgan must use all of his wits and Auban’s help to save his clan from annihilation.