Old water damage is almost always fatal to electronics so even if it does come back I wouldn't put a lot of faith in it (even if you aren't a pro photographer it would be a real shame to have it die during a trip).
For those who come across this thread in the future looking for advice on the same subject, I have rescued a number of electronic items from accidental immersion and purposefully subjected others to water when cleaning. A few pieces of information:
1. As soon as it gets wet, immediately remove all batteries (including the backup battery). Once all sources of power are removed you have stopped potential damage to components being inadvertently subjected to a source of voltage (or voltage beyond the design rating). This is CRITICAL and time is of the essence. Fresh, relatively clean water, isn't highly conductive but it doesn't stay fresh long as it picks up contaminants as it enters the camera and it mixes/reacts with other stuff in the camera so remove all sources of power and don't even consider turning it on to see if it still works at this point.
2. If it was only subjected to fresh water (i.e. rain) and not much of it, then it will probably respond well to drying. For electronics when there is only minor water exposure, I use a bag with desiccant and put it on a plant germination pad which provides a safe source of heat. If exposed to a lot of water, use a fan to blow air over it for an hour or so to get the worst of the moisture away or it will just immediately saturate the desiccant. Putting the camera or lens where the output of a dehumidifier is blowing on it is a good source of warm dry air for the initial drying of something that has been heavily exposed to water before moving to the desiccant step.
Good desiccant will change color when it is saturated and can be dried in a warm oven to dry off the moisture before reuse. I have some big bags (probably 3/4 pound each) of military grade desiccant that I bought years ago and I have reused it many times because it really doesn't wear out. Rice is far less desirable, it isn't a great desiccant and it can outgas "stuff" as it absorbs moisture which isn't what you want sealed in a bag with your electronics. Proper desiccant is inert over a reasonable temperature range and won't produce any corrosive vapors to further damage electronics.
When I don't have a lens on the body, I always keep a cover in place but if your camera needs drying then you don't want any cover over the sensor area and any doors (battery, card, etc.) and covers for interface ports should be left open to aid drying.
3. If you drop your camera or lens in salt water, the odds for success took a big hit but there is still a chance to save it. As before, immediately remove the battery(s) then immediately flush it with large amounts of distilled (not bottled drinking which has added minerals) water. It sounds radical but your best hope is to dunk it in fresh water, you can start with a couple of dunks in your tap water but finish with several immersions in distilled water before immediately going to two stage drying.
De-ionized water is even better but not as readily available and since cameras don't use high voltage (except for the the flash which isn't even that high for the built in flash) de-ionized isn't as critical. This flushing sounds radical but once it has been in salt water, you have nothing to lose and I have saved some gear this way.
After the fresh water flush (multiple times to drive out all of the salt) then use the standard drying recommended for a more typical fresh water immersion.
4. Rain can do more damage than most people realize. I shoot sports with 1 series bodies and weather resistant L series glass which are designed to be robust in a typical sports shooting environment but even then I keep the cameras as dry as possible and have the Canon combined lens/body raincoats. Consumer grade bodies aren't nearly as well sealed and many of the non-L lenses don't have rubber seals around the interface to the body. Rain itself generally isn't that "dirty" in most parts of the country but it will pick up the salt and oil that build up on the body from your hands and the rain will carry this conductive and corrosive stew into the body via openings like the buttons and rotary switches and encoders. So with a non-1 series body, try to avoid getting the camera wet even with rain and if you think it is likely to happen at the least clean the body thoroughly before you set out to remove the buildup from the body so that the rain won't be as contaminated when it enters the camera body.
5. One final note is a good camera bag is something you should always have when out in nature with your cameras. I use Lowepro AW (all weather) series bags which have a retractable rain shield built into the bag and a weather resistant Lowepro backpack for longer ventures. When going to sports events, the gear travels in waterproof Pelican cases. Even with this setup, keep some paper towels sealed in a freezer bag handy. These are just basic preparations because at some point you will get caught in the rain unexpectedly.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
