Photos of Mercedes Gleitze, the first British woman to swim the Channel, by E. Bacon, 1927; image of 1930 Rolex Oyster catalog via Time & Gems.

"Miss Mercedes Gleitze carried an 'Oyster' throughout her recent Channel swim. More than ten hours of submersion under the most trying conditions failed to harm its perfect timekeeping. No moisture had penetrated and not the slightest corrosion or condensation was revealed in the subsequent examination of the watch." —London Daily Mail front page announcement, November 23, 1927.