U cannot just change the socket to 20A. The wires must be 4mm too.

The power socket wire is 2.5mm only. Good for 13A max.

Better to pull 4mm wires from the DB straight to the heater. On the DB, use either 20A or 25A MCB. No need for plugs or sockets in between.

Much safer and reliable.

WH will require minimum 4 sq mm cable with RCCB 10ma protection  -  this is Suruhanjaya Tenaga rule!

Your water heater if on at full blast (max temperature)  will melt the cable and the 13A socket plug.

If you insist to use back the same wire you must minimally install the 10ma RCCB and make sure your RCCB is 16A at maximum otherwise your cable will melt and catch fire -  the cons is you should not on your WH at max temperature because your breaker will trip to protect your wire from overheating.

Still 13A socket point should not be used here,  go buy the one specialized for WH.  This is a matter of life and death though have fun arguing with some half-past-six electricians that will suggest with you to go with 100mA RCCB or even 300mA to prevent tripping -  30mA may be the best advice they could give HOWEVER 10mA is A MUST by ST rule!

Do not forget -  Insurance might choose not to compensate if they find that the accidents/fire was caused by wiring setup that did not follow the right specification!

Yes & no.

Very long time ago, when we wanted to install a water heater, the electrician tap from 1 of the lesser use power socket (13A) at the room next door. Controlled by a 20A switch.

So far 28 years, I'm still surviving.

Yes,  just make sure the professional is a real professional with licensed and advise according to guideline.

Life is cheap anyway and you are not alone actually, as I can see many houses are not setup according to specs and regulation.

The othet suggestion I have which are cheaper and do not require major rework would be changing the socket to the WH type 15A at least, and before the switch just install a dedicated RCCB 10ma  before your 20A switch which should give you a good protection already.

Yeap - if you can invest in this product is even better  - to replace the 15A plug.

You are welcome.

It doesn't matter... but I would recommend the 1st place just before the switch (outside your bathroom with the on-off switch together) - it would work to track the current movement along the circuits already. I don't recommend putting it in the wet area (inside bathroom especially) so I would not recommend option 2 which many would usually put beside it. It is not an IP65 66 or whatever IP rated device.

The instant water heater is normally at 3.3kW to 3.6kW power rating. So, current flow would be

3600/250 = 14A (worse case). Even 3300/250 = 13.2 A

The current 13A socket is not suitable. It still works for long time because there is buffer to its rating normally not specified. Therefore it works near its max rating most of the time. The rating is related to contact resistance between socket and plug. So, if the current is high, the heat will also generated thus weaken the copper property over time. The clamping contact worsen over time until the point it get burnt.

The best would be using higher rated socket and the best available is 15A socket. I've been using for more than 15 years with no burning sign and my heater is 3.3kW rated. As for the wire from the socket to the heater, the 2.5mm² is fine. It should be able to carry even 25A (or at worse 20A) at 60ºC ambient temperature, according to calculator I have here. Anyway, it is short cable.

The water heater has internal RCCB or ELCB, but test it regularly to ensure functionality. Adding 10mA RCCB provides redundancy just in case the internal one fail. Well, I don;t have one also 

On 2nd thought, since you have base box where you mounted your current socket, why not just straight away use 20A switch and feed wire directly to the switch. Just need to make hole on the base. This way you have double pole switch as isolation and no pin contact issue over time.

The water heater built-in ELCB does not to protect human, it is only for the equipment. This had been highlighted numerously by our sifu @ceo684 which is correct so you are left with only the main RCCB to protect you in case of any leakage outside the water heater ..and usually the main RCCB at 100mA would not be able to protect you from death... at 30mA you would stand a chance to survive but not one without severe pain and injuriy to your body.

This is also shared by Showertec