Funny thing, its rained a lot here recently. So a family member was on the way home from work, and found that the river was invading the bike path. We had a talk on the phone, and I failed to think very far ahead, which did generate some extra work and possibly lead to long-term damage to some bike components, but on the other hand, now I have had a new experience.
So the family member found a way through the flooded section that was acceptable for legs, but which saw the bike pushed through water that was way over the wheel centers. In fact this was an electric bike, and the electrical-connector-end of the battery went in the river, plus the motor, and all the control circuits. Did you know an E-bike could survive that? I did not.
Anyway the bike was apparently making some funny noises when it got home, but only while pedaling. I had also in the back of my mind the old Nexus 7 that a kid had (apparently) submerged in a forest, followed by neglect, events that accelerated the crapification process of that hub remarkably. So the wheel came off, the hub guts came out, and it was quite wet inside. I think a small amount drained out as I took it apart. So it got a long tour at the end of the boot drier before re-greasing and re-assembly.
The hub seemed a bit under-greased, perhaps that made the water ingress worse. This hub at the time of this writing is their latest Nexus 8 model, a SG-C6001-8C (coaster brake, regular bearings). So it ought to have the best available seals.
So the next day I went to assist this family member on the way to work, so I could see that the bike was OK. (It was after all an E-bike that got partially submerged, plus the hub I worked on.) We came to an uninteresting little underpass that had a puddle in it. Which was deeper than the wheel centers, this time on two bikes. (I wonder how many bikes that claimed in the past couple days.)
So when I got back home, I had front and rear hubs to think about. I wasn't really sure there was enough time under water to make a difference, probably under 10 seconds of submersion. This was however my favorite Nexus 8, a SG-C6010-8R. (It has the good bearings on the planet gears.) So I took it all apart, and sure enough there was some water droplets in there. Less water than the other hub, but not dry. It also got a tour with the boot drier.
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So anyway, approximately no submersion is acceptable for a Nexus 8, not even the latest models.
I don't actually know what the long-term effects of not taking action would have been. Some internal rust, certainly on the first hub. It also seemed low on grease, so it would probably have undergone rapid crapification. Even with intervention, I can't be certain that these two didn't retain water deep inside where it will rust in the coming weeks and months. (I have never taken hubs apart beyond exposing the innards as a unit.) I also do not know what amount of water ingress is tolerable for these hubs. They are designed to be used in wet conditions, and thats how I always use mine. I have never had an internal rust issue except for the Nexus 7 that took a bath and subsequently neglected for a year or so. That hub still lives in a kids bike.
I should also say that the front hub on one bike also needed new grease after all this bathing. The other hub... is an electric motor that shows no outwards signs of harm. Not something I am eager to take apart. Fingers crossed.
