When you go to charge your battery in the battery charger and experience any error when doing so or if the battery doesn't charge at all then the battery is most likely short circuited.
If this is not the case, then the motherboard has gone bad. The ICs are very sensitive to voltage limits. The saltwater most likely shorted many of these ICs.
Otherwise most likely the saltwater has corroded the various components of the camera and you would need to replace every circuit inside your camera. Therefore, your best option would be to buy a new camera.
Feel free to refer to our troubleshooting guide for further help with your Nikon camera.
Nikon D3000 Troubleshooting