Yes, water tap, even if you have only plastic pipes, the water between your tap and the pipes in the ground will only be 100's to 1000's of ohms, so compared to the 1M ohm resistance of the meter wont be a problem,
That or a concrete floor. (comparing ground at the outlet to effectively the house / plot of land around you)
Being a grounded plug you can also check the voltage between the laptop frame and outlet ground, by rights he should be 0V AC, if not the charger may be leaking current across the transformer (somewhat concerning)
Other questions:
PVC can be considered safe in the same way lead piping can be considered safe. if the water source does not contain components that react with the piping, and the piping does not decompose in to the water stream at a rate higher than found problematic in studies, then its safe for use. If you do hold fear, you run the tap for a bit before you drink to reduce the concentration,
In the nitty gritty it comes down to what plasticizes / additives where used in its production, being a thermosoftening plastic, they are generally less likely to seep out of the plastic unless heated.
For the grow lamp fitting, its more if someone accidentally breaks the globe, the goal is to be more likely to contact the ground than the live contact(s), the same as say a toaster, for a kid to pry something out with a knife, he will be in contact with the case, if its a RCD / GCFI circuit then him touching the case should trip it if he touches something bad, and for normal circuits the hope is that what you bridge the contacts with will trip the circuit, in the case if its your finger, then you may still get zapped,
For electric zaps, anything that stays away from your chest area is within certain limits, painful but survivable. live mains across your fingers, but with the rest of you insulated will hurt like buggery, but not near the heart so unlikely to cause problems, with you feet in the water, you main hope would be the current chasing the lamp fitting ground rather than one at your feet. as even if the puddle is not directly connected the ground it still makes a poor path through capacitive coupling.
The neutral is treated separately to the ground, It has a few functions, To make it so the dirt around your house is tied to the same voltage (without it, in lightning storms you can see hundreds to thousands of volts difference as the charged cloud passes overhead), To shunt any fault currents, e.g. knife in a toaster in the hope of popping the fuse / tripping the breaker. To trip a breaker that detects an imbalance of current, and more recently, to shunt higher frequency noise via some capacitors back to the reference point of the dwelling,
These capacitors however will leak some current as there impedance at 50/60Hz is not infinite. e.g. 10nF, so leaking in the order of 400uA,
And for the final one, a normal extension lead is at best able to withstand 1000V, with most high pressure sodium lamps running on the order of 2500V, I can understand it arcing.