if the socket have big switch and big neon light then it 20A but need picture for confirmation.

If you use lighting for heater sure will cause issue later. Not now, not a year or 2 but few years down, likely some issue start some crop like tripping or no electric supply at all.
Light is 1.5mm. Heater according to standard should be 4mm but many owner cry when see the price so most use 2.5mm wire. I personally will not provide any warranty for the job if owner want to use 2.5mm wire.

by the way, not sure if you know the difference between socket and switch :

socket
https://www.google.com/search?q=socket&clie...iw=1280&bih=686
switch(specifically water heater switch)
https://www.google.com/search?client=firefo....80.iT_sdKawugk

No need. I'll just post a link for you

Double Pole Switch for heater and aircond:
https://goo.gl/images/xvMDRz

Normal lighting switch with neon(not for heater or aircond). This is just a ordinary lighting switch but also with light indicator.
https://goo.gl/images/ecaCPm

if got 13A socket then i facepalm for you.
13A socket
https://www.google.com/search?q=13+amp+sock...iw=1280&bih=686
15A socket
https://www.google.com/search?client=firefo...1.0.3_P15oUlHTQ
anyway......i dislike water heater connected using a plug to a socket.

It is most owner who is calculative and want undercut stuff. 4mm is almost 20%-50% more expensive than 2.5mm plus the RCD deivce alone and when we submit a quote, they will say why we charge so much for simple stuff and request for cheaper price. Thats why i admit i don't get a lot heater wiring job as i wont back down on this kind of undercut way unless owner willing to COP or SIGN or any legal way to testify that i will not provide any warranty and be liable responsible from any issue arise from this water heater wiring. Which so far none agree too.

15-16A when running at max power.

cost depends........but can easily run up to rm100 i think.

electricuted or not depends on whether the ground is properly wired and working plus a working rcd for safety purpose.

undersized wire plus an oversized mcb will cause the wire to get hot and burn when being run exceeding the capacity of the wire for long period.

The guidelines for domestic wirings in residential buildings for standard 3kW water heaters are 20A mcb with a 10mA trip RCD wired to a 20A DP neon switch terminated to a ceiling rose or directly connected to the water heater wired 4mm2 in conduit from DB. You can replace the water heater mcb/RCD with a similar rated RCBO to save space in your DB.

Be prepared to pay around rm500 on average for the above. A wireman who qoutes less might be cutting corners or using existing wiring. Get the wireman to stamp in his PW number on his invoice to make him responsible for the installation..  to
This is just a general guide based our energy  commission (ST) for a standard 3 BR apartment. Ask to witness the insulation test, continuity test of the phase, neutral and earth conductors and the RCD trip test. The RCD trip time must be within 200ms to the nominal trip on the device.

A proper installation must be tested for correctness, safety and the client's peace of mind.

Just my thoughts to share..