I chose to go ahead and give it a try.
Result: The battery got hot to the point that I could not keep it in my hand for more than a few seconds, but not so much to melt the case.
Voltage dropped to 0.1 in about ten minutes, and that was the point where the temperature was at its highest.
The wires did not get as warm as I expected, probably because their resistance is negligible compared with the internal battery one.
Unfortunately I did not have a digital thermometer, so all of this test was very rough. A graph comparing temperature raise and voltage drop over time would have been cool. After dropping below 0.1V, voltage kept dropping, while temperature dropped as well - battery got less warm.
So to answer my own question: it was safe, at least in my case.
The author of the book, for what matter alkaline batteries, recommends wearing protective glasses (and I would add, gloves) just in case the battery is defective, so I advise to do the same in case you want to reproduce this.
If you do, and have more accurate measurement instruments (and patience) than I had, it would be interesting to see the behavior pictured (and compared with a test with an alkaline cell.)
By the way, in the minutes I took to write this the battery is still providing current into the short circuit. It is at 45mV right now.
It's hard to generalize to a battery type because any poorly made mass production battery has the risk of catastrophic failure in high drain situations just because of poor quality control.
Having said that, it is generally my experience that NiCd and NiMh perform quite superiorly in high drain situations compared to alkaline. This is almost entirely due to their lower internal resistance compared to alkalines. The higher the internal resistance of a battery, the harder it is for the battery to provide higher currents and the hotter it will get when it provides those currents. When the battery overheats, it can vent and warp and leak and lose capacity.
Many of the hybrid vehicles use NiMh batteries and they are used in a very high drain situation; yet the Prius for example has had very few battery failures in its 14 year history.
I have been using high quality nimh rechargeable AA and AAA in my high drain flashlights for a couple of years without a single failure. I suspect you have just had bad luck with your rechargeables? Or possibly let them drain too far or over charge them which could lead to the catastrophic failures.
Best Answer
Whether a particular battery cell can survive under water has nothing to do with its form-factor or its chemistry. Most batteries are not designed to operate submerged under water. No matter what size, shape, or chemistry they are.