Ginty just pulled the whole "and now I shall forever think of Cousin Jon" stuff that made Karen snap at her too. So between her initial refusals and Rowan bossing her into it, she'd managed to piss everyone off to a fairly epic degree. Rowan is also such a (literally as we find out) get-back-on-the-horse type, I'm sure she thinks Ginty should go in swimming and diving to redeem her Kingscote failures.