My immersion heater switch gets hot and crackles - why?

what is an immersion heater? where are you putting it? how hot is it getting?
does it cost less than $20?

Don't use it til the electricians are here! Sounds like shorting wires in the switch that could start a fire. Maybe just burn out or blow a fuse, but still.
What brand is it and how many watts/volts?

Okay, are we talking about an industrial immersion heater or a household immersion heater, such as one you would use to heat the oil in a household deep fryer?

Apart from that, the crackling and the switch getting hot are very bad signs. They indicate that the innards of the switch have loose connections. The crackling is most likely the bad connections bouncing together, the heat is caused by the contacting points arcing (sparks).

Either replace the switch or get a new immersion heater (smartest thing to do)

Maybe the type you sticking a cup of water? Toss it.