What does Nick metaphorically compare us to in the closing lines of The Great Gatsby?

I agree that yes, Fitzgerald is comparing us to boats, but
if you read the paragraph from the line before the last, it helps put it into
perspective

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Gatsby believed in the green light, the
orgiastic future that year by year recedes before us. It eluded us then, but that’s no
matter — to-morrow we will run faster, stretch out our arms farther. . . . And one fine
morning ——

So we beat on, boats against the current, borne
back ceaselessly into the
past.

Essentially, it seems
as though Nick looks at Gatsby's pursuit and failure and says, "But hey, aren't we all
like that? Trying to reconcile things in our past to our present, trying to revive
the dreams we loved, but lost?" I think Nick is comparing us all to Gatsby, pursing the
things we want because it's in the American culture to believe if we try, we will
succeed. And if we fail, we must try again. And while we will often times succeed, we
are all fighting with some aspect of our past that want to redeem in our
future.