As you know water and electronics and electricity are not a good mix.
The water (especially salt water) causes corrosion and provides circuit paths for the electricity which were not in the camera's operating design and could damage the components.
First do not turn on your camera and then you need to with the camera turned off, remove the battery as soon as possible from the camera to minimize further damage.''
Then you need to dis-assemble the rest of the camera and clean all the affected parts using Isopropyl Alcohol 90%+ (available at most pharmacies), to remove all traces of corrosion and water. Do not use "rubbing alcohol" as in some cases this is sometimes only 70% IPA or less, can contain scents and is not as effective. If you do check the label to verify the amount IPA
Here is a link that describes the process.
Electronics Water Damage
As always with electronics, especially surface mounted PCBs be gentle when handling and especially when brushing away the corrosion. You do not want to remove any components from the board.
Hopefully after you have done all this the camera might possibly work correctly again.
Here is a link to the ifixit guide(s) for repairing your camera. GoPro Hero3+ Black Edition Repair (I couldn’t find a repair guide for your exact model, hopefully this one is close enough to be of some help)
If this process seems too daunting, take your camera to a reputable, professional camera repair service, experienced in water damage repair and ask for a quote for a repair. If you decide to do this, do it sooner than later.