Lines 66-76 Summary

Get out the microscope, because we’re going through this poem line-by-line.

and victory filled up
the little rented boat,

Victory seems appropriate for a fishing story, right? But who's victorious here?
The victorious feeling is obviously overwhelming if it fills the entire boat.

from the pool of bilge
where oil had spread a rainbow

Bilge is the water that gathers at the bottom of the boat near the engine, usually mixed with oil from the engine.
So the rainbow is coming from the spilled oil, just like spilled oil from a car engine in a parking lot or garage.
The way Bishop words it, she's saying victory is coming from the nasty bilge water where a rainbow has formed.
This is the same combination of nasty (oil) and beautiful (rainbow) that Bishop has been creating the entire poem.

around the rusted engine
to the bailer rusted orange,

We get more color, and now maybe we see why Bishop includes the rainbow.
Again, we see the vibrant color against the rusted old engine. This contrast is sort of predictable at this point, but still effective.

the sun-cracked thwarts,
the oarlocks on their strings,
the gunnels, until everything

was rainbow, rainbow, rainbow!
And I let the fish go.

Well finally all the colors of the poem come together in this very enthusiastic, triple rainbow ending.
Remember that the feeling of "victory" came from the rainbow. But who was it a victory for, the speaker or the fish? Bishop lets us decide.
The speaker lets the fish go – either because catching the fish feels like enough, or because the speaker has too much respect for the fish and counts this encounter as another getaway for the old guy.
Who knows? What we can tell is that the struggle between the speaker and the fish seems to culminate with the rainbow, and then calm down with the release of the fish in the final line.