Tress of the Emerald Sea by Brandon Sanderson

The only life Tress has known on her island home has been a simple one, with daily pleasures of collecting cups brought by sailors from faraway lands, and listening to stories told by her friend Charlie. When his father takes him on a voyage to find a bride, disaster strikes. Tress must stow away on a ship and seek the Sorceress of the deadly Midnight Sea. Amid the spore oceans where pirates abound, can Tress leave her simple life behind and make her own place sailing a sea where a single drop of water can mean instant death?