Is it on the boat or off? If it is off, and a single piece coupling (not a split), you might want to replace it and have the new one it fitted and faced to the shaft. More often than not when removed you loose your tolerance (the rust layer) and the fit becomes sloppy which can lead to bad issues down the road for your shaft. If it goes back on and does not require a light press fit the tolerance between shaft and coupling is gone.

If it is still on the boat a drill with brass brush wheel or a Dremel with the Scotch Brite wheel will work well.