Al states that watching Marilyn Monroe skinny-dip is like looking at Helen of Troy or Botticelli's "The Birth of Venus".
Helen of Troy was a Greek mythological figure (historically inspired) that was one of the causing factors of the Trojan War. According to the myth, when Paris, prince to Troy, fell in love with the renown beauty, he brought her back to his home, stealing her away from her husband, who, in response, began the subsequent invasion of Troy to get her back.
Sandro Botticelli's The Birth of Venus is a famous painting of the greek myth concerning the birth of Venus, the Roman goddess of love, who was supposedly conceived from sea foam.
Both women are considered to be great beauties, and their appearances are historically, culturally, and socially recognized as such.