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.