A recurring theme in the books set in Brittany is that it is impossible to go bathing at low tide because the sea simply disappears. In Breton Adventure, Sara decides to test whether this is actually the case, as she finds it a little hard to believe. In Chapter 2, They Meet Artichokes, Ajax - and Raymond, it is stated that:
"Sara, who liked to prove things for herself, declared that they ought to put this to the test: Caroline thought it was probably a waste of time, but wanted a bathe anyway, so they put on their bathing dresses at the grève, although the sea was not actually in sight. They walked hopefully over wet sand for what seemed about a mile, and less hopefully over sand and sea-weed for another: they did finally reach the sea, all right, but as it obstinately refused to become any deeper than knee-high, they gave up in disgust, before they reached home and mother, Caroline said. And Sara thereafter was more ready to believe that when Madame said there was no bathing, there was no bathing."
However, at low tide, the Roches de Saint Quay islands appear, allowing the characters to go on la pêche, or shrimping.