I don't know what kind of motors are used in that type of vehicle shown in the video clip.

However, the motors must be weatherproof; they are not kept completely dry all the time. If the vehicles use the Düwag momomotor bogies, the whole set must be sealed, because the motor block also contains the gears.

About electrocution: as long as there is contact to the ground through the wheels, there is no risk of electrocution when you are in the train. Even if the grounding no longer works, there is no risk as long as you stay in the train. You may be at a potential different from the ground, but the whole vehicle is at that potential.

Risks may come up when trying to get off, because then you may make the only connection to the ground.

So, that union representative simply makes in FUD when talking about electrocution or so.

However, it is definitely a question whether it was smart to run through such deep water. You will get dirt into the vehicle, which means you will have to dry it out and clean it. The mentioned rules that you may proceed at low speed as long as the tracks are visible in the water does sound reasonably to me.