Only the middle one, Maxguard, is the correct specifications for a water heater at a sensitivity of 10 mA or 0.01 A

Sorry, never used before. There's something you must realise which unfortunately is this: Contractors don't recommend something based on your best interest. They recommend something because it is easy to purchase, or that is what they have in stock, or it has a large profit margin, etc.

You also cannot trust something simply because it has a SIRIM sticker. You must get something that has a SIRIM sticker that has passed electrical specifications. A company could very well get a SIRIM sticker on the claimed weight of the item.

That being said, if funds are limited, then go for Maxguard, EPS or Himel branded RCBO with these specs : 20 A overcurrent, 10 mA trip. All have SIRIM and are JKR approved*, meaning they have at least been electrically tested by someone independently. If you go for an RCCB instead of RCBO, then get 25 A, 10 mA trip. The 25A is because there will be an additional MCB for the water heater. NOTE: these specs are if your WH is below, say, 4500 W, which it probably is. Please review if the WH happens to be above this wattage.

I can tell you that the store "Combi Electric" online delivers very fast, and price is fair. Now, saying that "Contractors don't recommend something based on your best interest" why should you trust me? I have absolutely no connection to the store just stated, I am not making money off you, and I am only recommending this based on your safety, because I can't get anything else from you!

Disclaimer: * last time I checked

This is the reason a 10 mA RCCB/RCBO is specified for water heaters, either instant or storage, whether your heater is Grounded/Earthed or not. You become the Ground/Earth point, and the RCCB/RCBO trips before a fatal current electrocutes you.
As for this, you should absolutely also use isolation barriers (basically, a section of non-conductive pipe to break the path of the hot water copper pipe), or use PP-R or PB pipes for hot water.

ElectroBOOM shows the effects of even a small gap of water on electrical conductivity on YouTube https://www.youtube.com/watch?v=dcrY59nGxBg&t=93s

Sorry to be a bit pedantic, but I have to point out...
ST = Suruhanjaya Tenaga = Energy Commission = can make rules that must be followed
electrical and electronics association of Malaysia = guidelines, no legal standing

Are you saying ST lowest RCCB sensitivity for lights and fans is 100 mA, or are you saying ST requires metal awnings and metal roofs to be electrically grounded? The former is true, but I have never read the latter. Could you please cite a source if you mean the latter?

Thanks! Most appreciated!

Looks like ST has finally published an updated guidebook (Dated 1st December 2021) covering a lot more information. Lampiran III and V are particularly important (Pages 59 and 61) as it covers the current carrying capacity of wires, and is way more conservative than any manufacturer tables. From the ST table, 2.5 mm² cables no longer are a 20 A MCB circuit....