To operate a water heater via the HP/HC switching signal provided at the meter you require the following:
A maximum 20A disjoncteur for the actual water heater load circuit (which must be in 2,5mm² conductors).
A 2A disjoncteur to protect the switch circuit that runs from the main tableau to the meter & back (uses 1,5mm² conductors)
A "contacteur jour/nuit" to respond to the switch circuit & to do the actual power on/off of the water heater.
There's a diagram here.
There is no need for a timer as the grid/meter does all that for you.
If you are using a heating programmer (such as a Delta Dlore Calybox) then the 1,5mm line to the meter is not used & instead you use the "téléinfo" contacts on the meter to run the programmer digitally via a screened cable. The water heater hardware works the same though.
More info about that here.
All the above only works if you have the right electricity account (heures creuses/heures pleines) & a "double tarif" meter, which used to be a different item in the days of mechanical meters but nowadays is just a question of having the digital meter reprogrammed. Your bills will say what you are subscribed to if you can't work it out from your meter display.