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.

I understand perfectly how ELCB and RCCB work. Why the big name in water heater manufacturer still use ELCB not RCCB? ST should make it mandatory to use RCCB for water heater, don't they?

How did people get electrocuted in cases of water heater? The casing is plastic and the hose is silicone rubber. Is it because the water heater uses ELCB not RCCB?

The main problem with ELCB is more into the functionality of Ground wiring which maybe overlooked. The L and N wire if no connection the user will know immediately since device will not operate but for E failure no one will notice. This will not trigger ELCB in case the E circuit failed.

The new device that is more advance than the RCCB is GFCI. This will performs what RCCB does and also sense short between N and E. My company is producing this for a Europe customer.

Well, many electrical contractors charge too much but using low quality material. Most likely it is because they need to pay workers salary all year round and projects are not continuous all year long. For that reason, I also prefer DIY

I'm not sure about the price because I'm not in this business line. However, if you talk about the manufacturing cost, ELCB is more expensive to produce compare to RCCB but the RCCB perform better than ELCB.

I guess the main reason for ELCB in appliances is detect faulty appliances

Products made in Europe (if it really is) are expensive because of manufacturing cost there. Even when there are similar product from China, they would buy Made in Europe. Nowadays due to component shortages, many Western product s are also using components from China. This is very common now. Those US companies producing their products in China is shifting to Malaysia too. Well, labor in China nowadays are comparable to Malaysia, I'd say, but other cost is higher because components are still from somewhere else.

Many Europe companies are assembling their product in Europe but but parts/modules from Malaysia too. Component from China are also used for their low cost. The CE certification is not like UL in the US. It is the manufacturer that declare and responsible to meet the CE requirement. It is not tested by lab in countries that are importing the products or at least CE certified lab.

The branded product is also expensive because the insurance they paid to protect themselves against any possible lawsuits too . Western manufacturing agreement is a stack of papers while China only need 1 piece of paper. This was true story from my American ex-boss. My work now is estimating cost to produce a product based on BOM list and drawings provided by possible customers and the selling price difference would surprised most people.