Gather Close by Megan Derr

Rosa is the eccentric daughter of the royal family of Seavow, the greatest of the Kraken clans. More interested in combat that conversation, and named for a plant that doesn’t even exist in her world, she has always stood apart.
Her chance to change that comes with the unexpected offer to marry Lord Gaston, the notorious favorite of Queen Marga, often called the Queen’s Man. Even though everyone knows he has a lover, a beautiful Sylph with plenty of scandals of his own to his name. Even if she succeeds and the marriage goes through, she will still always be the second best cast to the side…