Hi Bob,
I have my personal experience when I detected a shallow creek with my 2006 R5 T2. Shortly after that, it began to false constantly, even when held up in the air away from any metal. The factory wanted me to return it for destructive analysis and while my machine was there, I had the R6 upgrade performed and it was returned with a new coil and the upgrade.
I'd say I've seen maybe a half-dozen other similar reports. That's not that many for however many total units being sold. Not everyone who may have a bad coil will report it, while there seem to be many users who do post who have not complained.
To try to draw a conclusion from this admittedly limited perspective, I'd say there are a few leaky coils out there, but probably not that many in total, or we'd be hearing much more about it.
If you intend to hunt where the coil will be well-wetted or immersed, you have two courses of action. One, test it to see if it leaks and return it for an exchange if it does, then test the replacement. Most coils would appear to pass the test. Option two would be to seal the coil yourself so you're satisfied it's leakproof. Would doing this be called tampering and void the warranty? I don't know, since any sealant would be applied to the outside of the coil housing.
So far, my replacement coil hasn't given me any problems, but I guess I'm more careful now to keep it dry, so until I give it a dunk test, I can't honestly say either way. I'd be one to seal it myself and not worry about warranty issues. But, that's just me! I think I'd look most closely at where the cable enters the coil housing and the little separate cap surrounding the cable entry. With an open web coil design, there are more inches of seam in the housing, so it probably wouldn't hurt to seal those also.
Once a coil with a leak dries out inside so that it behaves normally again, it can be sealed and put back to work.
I sure don't have access to any factory data, but I'd suspect they should be aware of the issue if it exists to any great degree. I only have my own experience and what I read right here on the forums. Loose cable wires can be determined by experimentally flexing them or taking continuity readings with a multimeter. If a coil gets wet, then acts flaky shortly after, I tend to leap onto a probably leaky coil as the diagnosis.
-Ed