Danger of electrocution is not particularly high, but can easy start a serious fire with such batteries. The energy capacity with create a seriously hot arc if shorted, and there is a potential for thermal runaway of the battery cells.
I once saw two marine lead-acid batteries explode and catch fire in the boat next to mine when the owner had dropped something big enough to short circuit and not disintegrate itself. Instead a fat copper cable exploded and then probably both batteries boiled, exploded and then everything caught fire. It took about 3 seconds.
Luckily he avoided the acid spray, but he had to empty 4 large fire extinguishers from the nearby boats to put out the fire and cool down the remains of the batteries enough so they did not immediately catch fire again.
I have a huge respect for what a huge solar array and a big bank of batteries can do after seeing what a relatively puny battery could actually do.
A standard AC system can't deliver even a percentage of that current.
I had something similar happen to me in a Datsun 180B once.
Driving home in the rain, and the tail light wires were submerged in water that was pooling in the trunk and shorting out. As I put my foot on the brakes - on the freeway in the pouring rain - the fusible link in the engine bay decided to just catch on fire instead of being a fusible link. The battery quickly boiled and I had a sudden and catastrophic engine bay fire on a crowded road.
A car battery has enough amps to literally use as a welder (two jumper leads, an arc welding rod and a 550CCA car battery is enough to stick most things together in an emergency :P ), and is more than enough to create a massive fire.
Yesterday I was putzing around with these exact batteries (swapping some old tooling that had great build quality but were NiCad junk). Those 18650s pack a punch as current sources. I had an ammeter hooked up and we're talking easily in the 10s of amps for a brushed DC motor of your standard cordless Milwaukee from the 1990s.
I did the same to fix one of those $40 dollar Swiffers (i.e., we're not really talking about powering industrial steel chop-saws powering through I-beams). It drew ~2.5 amps at no-load, no problem.
OSHA[1]:
"17-90mA" : Death is possible.
"90mA+" : Death is likely.
And remember breakers blow only when the sub-circuit's current exceeds the threshold (generally ~15amps in the US) so you can spec a standard gauge of wiring sufficient to consistently draw that current, not overheat and burn your house down. It's (generally) not checking to see if the current sunk = current sourced. You need GFCIs("RCD"s in other locales) to actually get that functionality.
The rule of thumb I've heard is 50mA passing through your heart is where the v-fib dangers begin. The real risk begins at as low as 20mA, because thats when your hands lose their muscular control. You can't let go of the source, and your heart enters v-fib (~100 mA) and you get oxygen starvation and brain death[3]. You have tales of people using one hand to support themselves on a grounded breaker box, using the other hand to just have a look-see, and not even realizing they're in v-fib, feeling a little 'off', sitting down for a bit to catch their breath and found dead 20 minutes later next to the box.
Your body basically acts as a resistor in parallel with the circuit. Here's the ASME's resistance model of an adult males' body[2]. As you can see, you're body is definitely not a 10meg resistor. If you're working in damp, humid, or hot (i.e., you're perspiring through your hands, and that Na+ is just looking for a donor electron!), the risk increases. Whether you're working on a 460v3ph 15 horsepower Hardinge lathe or a Swiffer, the best piece of advice I ever got was "have a healthy respect (and fear) for your tools". Use isolation transformers, current limiters, and CAT-rated gear and read a sufficient amount of information to inform yourself in advance of the potential risks[4].
[3] Brain death I've always heard is ~5 minutes for an average adult male with a decent pulmonary system and respiration capacity-- less if you're a couch potato with less lung VO2 capacity, more if you're Lance Armstrong. Obviously not a physician.
[4] I was googling to pick find a diagram to depict the resistance model of a human and stumbled across Allaboutcircuit's safety text (chapter 3, for those who are wondering). It's pretty good for "general" electronics. Obviously if you're working with things like tubes ('valves'), transformers, and other subsets, you're going to want to read about the safety precautions you want to take for those specific areas as well.
