If electrocution is your concern, please check your DB to see if you have 10mA RCCB installed for every heater. Else, it doesn't matter which SIRIM certified brand you choose as the risk of upcar is still high.

Iinm, most shops only sell 25A 10mA Sneider RCBO. Supposing he's using 2.5mm² wires, will there high risk of wiring overheat especially if the wires are old?

2.5mm² isn't too small. I checked with an M&E consultant. Continuous current should not exceed 20A which is within the spec of most water heaters. Plus, if paired with 20A mcb/rcbo, it protects against overcurrent.

There are 2 issues here - over current and current leakage. A 25A overcurrent protection for 2.5mm² wires looks unsafe to me, regardless of its sensitivity.

Maybe that's why he mentioned Schneider RCBO. I was also trying to find a 20A 10mA Schneider RCBO but all I can see is 25A version sold locally. My electrician said 20A version needs special indent.

My house uses 2.5mm² conductors for WH and Aircon so I am particular about 20 vs 25A overcurrent protection.