Water heater cannot use plug. It will melt.

Opt for 4mm wires and connect directly to the MCB.

I have seen cases people using 15A plug and it melted the socket too.

So, don't play2.

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.