The Mermaid's Shoal - The Forbidden Sea, #1

No-one leaves these waters alive... Elfyn O Se has lived by the laws of the sea his whole life. He'd go by Elf to his friends if he had any left, but at least his crew are willing to put up with him, and it's not because he accidently cursed them all. Yet, maybe he can pull them together for one last job that might ease the fury of the tyrannical mermaid he wronged so long ago. He might have a chance to break the binds for himself and his crew, if he can pull it off.
There's just one problem; a rival mermaid called Anwen. As mysterious as she is terrifying - and hiding secrets of her own - she offers an alternative to break the curse if the crew can help her get home. The crew of the Ossory now finds themselves caught in an odyssey brimming with magic and monsters. Battling against champions of terrible adversaries, mysterious lights from the deep, and kleptomatic selkies who deal in blood, Elf is faced with a choice.
Will he risk the future of his world to save his crews' souls, or condemn them all to protect the archipelago from certain calamity?