Adding extra socket share with water heater power supply

[JJ1]

It is safe to add on extra socket by this way?
Original connection:
postimg.org/image/v6nshoygh/
Add-on connection:
postimg.org/image/vkoccsyu5/

The extra socket is for light usage only. Water heater is instant water heater without tank. The switch is double pole single switch.

 

[JJ1]

Please give me some advise.

 

[Averagesupernova]

That drawing really doesn't tell us anything. If you have to ask, you shouldn't do it. If you have a reference book that explains something like this well and you completely understand it then go ahead and do it. My bet is that you will not. A circuit that feeds a tankless water heater will be too large to safely run a light circuit. In the USA we never interrupt the neutral so right off the bat you are wrong (if you are in the USA) with a double pole switch as I am assuming you will use one pole to switch the neutral and the other to switch the hot.