Electrical – How are electric water heating elements isolated

If isolation isn't specified in the datasheet then I wouldn't assume it's there. Most likely both FETs are on the same substrate. That means they are sortof dual diode isolated, but the breakdown voltages of those diodes probably isn't much more than the max D-S voltage allowed.

In any case, that's just guessing on my part. If it's not in the datasheet, then its not in the device for practical purposes.

When the earth is dry. sometimes, earthing works badly.

A pathological case for earthing is things on top of mountains.

The mountaintops tend to be pretty dry. People like installing observatories atop mountains. In a past life I worked with observatories.

The earthing rods are laid out in a ring around the observatory with a buried cable linking them. The rods have tops high in the air; point is to get some lightning protection, and an earth that works at all. (Lightning likes high, metallic things like observatories.)

The air is very dry on top of mountains. ESD is normally prevented by earthing.

Standard protocol was for staff to urinate on the earth spikes instead of using the porta-potty (whenever possible without upsetting tourists).

To make it harder, many sites share the mountaintop with radio transmitters. All that radiated RF is hard to screen when there is no functional earth to connect to. (Radio guys, you have the same problems but you caused mine dammit!)

A co-worker had similar problems earthing in a past life with sand dunes and earthing. Getting the septic tanks to drain on to the dunes can help.