What does it take to damage an internally geared hub?
Usually jumping curbs isn't a problem, as long as you're cognizant of not landing incredibly hard.  I would think that rough landings would damage the rim before the hub.

Most internal hubs wouldn't shift under torque at all.  Some have a system where they won't even try to shift if the mechanism is moving, others have a "sprung" system where it waits for a pause in pedaling and makes the shift then.  

Brief submersion shouldn't be a problem.  Prolonged would most likely cause big problems.  I don't know if shifting while submerged would exacerbate this...

The biggest factor I've seen in damage to an internal hub is using one that's slightly mis-tuned for a long time.  I'll have someone come in saying that it stopped shifting into third about a year ago, and now it won't shift into second...  By the time they bring it in to the shop, now it's a much bigger problem than it would have been a year ago.