"So we beat on" shows that they move forward, but "boats gainst the current" shows that they do so in struggle, which relates to "the past." Sounds almost like a cycle, like how the future is always connected to the beginning or past events. When moving forward, you may fall back to the start of your jouney after a great fight or struggle. In terms of the novel, Gatsby struggled to make something of himself and becoming someone great but it all ended when he died, leaving nothing great about him, going back to the start.
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