The persona in "Wild Nights! Wild Nights!" is a lover--we'll assume a woman--who yearns to be united with her beloved. The liaison is imagined, not real, because the subjunctive "were I with thee" is used. Dickinson uses imagery and metaphors about a boat at sea in the poem. The second stanza speaks of emotional intimacy, declaring that once one's "heart" is "in port," the effect of wind is "futile." This means that, with the two lovers committed to each other, no outside forces can break up their emotional attachment. 

The final stanza builds on the second stanza and can be interpreted as reiterating the emotional intimacy already described and/or going on to speak of physical intimacy. The metaphor of a boat moored at a dock creates the understanding of physical union: the boat and the dock are physically connected to each other by a line. The metaphor is one of sexual intimacy where the speaker, represented by the boat, is one with her lover, represented by the dock. To be moored "In Thee" is to be physically attached just as a boat is to its mooring place. This creates some irony with the first line. The sexual activity is what makes the "wild nights"--but the boat at its dock is not wild, but safe and secure. The "Heart in port" makes the "wild nights" just like "rowing in Eden." The lovers' paradise involves both emotional and physical intimacy. 

The key to this metaphor is the word moor, which means to secure a boat or ship securely. The speaker in the poem indicates a yearning to be anchored or soundly attached to another person. In the earlier stanzas of the poem, "Wild Nights--Wild Nights," the poets includes references to ships and sailing: "a heart in port," "compass," "charts," "rowing in Eden," and "the Sea."