Thou counterfeit'st a bark, a sea, a wind; For still thy eyes, which I may call the sea, Do ebb and flow with tears; the bark thy body is, Sailing in this salt flood; the winds, thy sighs; Who, raging with thy tears, and they with them, Without a sudden calm, will overset Thy tempest-tossed body." What is the extended metaphor in these poetic lines?