One needs the proper attire. I was informed of this by Phoebe and Rowan. Proper attire is, in the case of these two strong minded girls a bathing costume (green with spots in the case of the former and a rather lovely floral tankini in the case of the latter), a pink fish swimming hat and a pair of goggles. Alas I only have photographic evidence of Phoebe in her outfit:
I must say though that Rowan pulled it off better looking like a young Esther Williams in her bathers and hat.