Can I do anything about a Nikon... (with) saltwater damage?
Since Nikon does not sell internal repair parts to other service centers or to end users, and does not repair water damaged cameras themselves, your options are very limited. Beyond a simple disassembly, cleaning, and reassembly from a third party repair service there's not much of anything you can do. Perhaps if a part needs to be replaced you might find a shop with a parted out camera that contains the part your camera needs.
Some might cite this 2013 Petapixel article as evidence that Nikon will service saltwater damaged cameras and lenses. However, the "Nikon Service Center in Taiwan" that did the repair shown in the article is not an actual Nikon authorized service center. As the home page of their website clearly states at the bottom of the page [retrieved on 6 March, 2018] :
NRC CO., LTD. ©2015 All Rights Reserved.about me
We are not an official service provider for Nikon Corporation
Further, since 2012 Nikon has stopped selling replacement internal parts to non-affiliated service centers or end users. They did start selling external parts only, such as grip covers and battery doors, again in 2013.
Unless the shop still has a part in inventory it already had in 2012, if such a repair were attempted today the necessary 'new' parts would need to be cannibalized from another copy of the same lens, possibly only from one from the same production revision and running the exact same version of the lens' firmware.
According to Roger Cicala, the founder and overall technical guru at lensrentals.com¹, saltwater damage is more devastating to cameras and their internals than just about anything the gear his company rents encounters on a regular basis. He covers it in depth in this blog entry, but he has also mentioned it in many others.
¹ Probably no one in the world oversees a larger inventory of cameras and lenses that are used to take photos, rather than being stored in a warehouse as inventory to be sold, than they do.
At Roger's company, they don't even part out unrepairable cameras with salt water damage due to the concern that there may hidden corrosion in those parts. Normally, those guys part out just about everything - even some full frame cameras with a single scratch on the sensor get parted out to repair other cameras in their vast inventory:
Lensrentals insider joke: What do you call a D800 with a scratched sensor?
... Parts. Because at $1,800 for a sensor replacement . . .
But in the case of salt water damage:
But the amount of salt and corrosion here and on the bottom means we wouldn’t trust anything in this camera, ever again. It can’t even be a parts donor — the chance that those parts will eventually corrode and fail is too high. That’s why many service centers won’t repair water damaged cameras; they have to give a warranty after the repair and chances are very high something they didn’t replace is going to fail during the warranty period.
I would be surprised if Nikon factory service would even repair a camera with internal saltwater damage, since they'd then be on the hook for additional repairs within the guarantee period as other internal components subsequently fail.
From the same Lens Rentals Blog entry cited twice above:
Most service centers won’t work on a water damaged camera, even if you pay them. Some won’t even open it up to look inside if they see evidence on the outside.