That's what I thought at first, but I don't think light switches show red on the toggle when on, only higher rated switches.
I did wonder if the immersion heater was starting to short due to corrosion, but the Experts say the sparking is normal.
When breaking a live circuit there will always be some element of arcing and sparking until the contact gap is sufficiently wide to stop the current flowing.
Think about how lightening, or spark plugs, or gas hob ignition etc.. which illustrate the principle that electricity will jump across gaps when specific conditions are met. (And the advice if you smell gas do NOT switch anything electrical on or off).
Basic rules are:-
Greater loads = larger potential sparks.
Cheaper switches = more visible sparks.
Smaller loads and better quality switches = very difficult to see the spark, (but almost certainly there will be one).
Older switches with a more mechanically robust toggle operation and thicker and darker bakelite construction, typically made it difficult to see any sparks, and early white plastic, "UK manufactured", switches also concealed any sparks produced during normal operation...
However many modern switch brands typically have cheaper quality switch mechanisms and contacts..
and thinner plastic moulded front plates, so any sparking can be more visible especially if the room is dark.
