Hi adrianvon,
please let's be realistic.
Water heaters are normally designed for 1KW, providing hot water for a single shower.
There are 10KW water heaters on the market, but they are all three phase devices.
So a 1KW heater would require ~4.5A at 230VAC.
Solid state relays are designed for different AC voltages with and without zero crossing unit built in.
Here is an SSR I suggest for your application: SHARP S202S02, rated 230V/8A with zero crossing unit.
The LED forward voltage is 1.2V and the forward current is 8mA - no reason to use a mechanically latching relay since the heater will be switched on and off within short time intervals.
Using an SSR it's advisable to use a 0.6W VDR across the AC terminals to avoid back firing into the built in triac.
Have a look into the attached datasheet.
For higher currents look into the second datasheet of Carlo Gavazzi SSRs. They should do for three phase high current heaters. Carlo Gavazzi SSRs are internally fitted with a snubber network.
Boncuk