usually no need to brake tiles. just pull the wires through the pvc hose
should use 4mm2 cables. (my mistake. should not be 2.5)

https://www.st.gov.my/en/contents/publicati...r%20Systems.pdf

you may read the guide cover to cover.
some points
- use double pole 20A water heater switch
- as much as possible, the LN wires should not have joints.
- earth cable should not have joints.
- do not use 3pin plug to connect water heater
- leave a loop of wire before entering water heater (so water wont go inside)

Yeah usage of 3-pin socket + plug connection to power up water heater is a BIG NO for water heater, according to Suruhanjaya Tenaga guideline.
U can either

a. get the contractor to remove the 3-pin socket and its junction box and connect the electrical cable directly to the water heater, which u might need to hack some walls to hide the cable and to fill / cover the hole left from the removal of the junction box, or

b. a cheaper and faster option is just to replace the 3-pin socket with a water heater connector (shown below) while reusing the 3-pin socket's junction box, so no hacking or filling holes required. U can easily purchase the water heater connector online - Shopee / Lazada