Metaphorically, Nick compares us to boats that beat on, pressing on against the currents that can be rough at times. Nick believes we are often stuck in the past because the currents hold our boats from going forward into the future. Nick believes that Gatsby lived in the past and could not be free to move forward into the future. Daisy kept Gatsby held in the past. He could not get over loving the Daisy he once knew. Nick realized that Daisy had changed, but Gatsby continues to love Daisy. Gatsby died living in the past. His boat beat against the current and was pulled back into the past. He dies loving Daisy:
So we beat on, boats against the current, borne back ceaselessly into the past.