Is my water heater connected to GFCI and is it affected after short circuit?
Find the Fault

Firstly, the problem is almost certainly not your GFCI or Circuit breaker.

What has happened is that a fault has developed in your water heater or in your power outlets or someone has hammered a nail into a wall and punctured and shorted a wire. The circuit-breaker has then done the right thing and isolated the circuit to prevent your house burning down in the night whilst you are asleep.

You need to find that fault.

Lights

Your lights are on a separate circuit, that's why they are not affected.


Fuses

Your main distribution board looks like it is fitted with fuse-carriers (not with more modern MCBs or RCBOs). You need to open up the relevant one and check the fuse inside for continuity.

It isn't clear to me how your subsidiary distribution board is connected to the main distribution board. In my part of the world, a subsidiary board is fed from one of the fuses (or MCBs or RCBOs) in  the main board - which are labelled to make this clear.

In your case it isn't clear but I'd turn off power to the building and then check fuse 6 since the symbol on that looks a bit like a water heater.


You should only replace this after you found and fixed the fault in your heater room.

It might be that the overload in the room with the heater blew the fuse in the main distribution board as well as tripping the MCB in the subsidiary board (the one with the 0/1 switch). That fuse probably supplies power to the subsidiary board which divides it between the heater and sockets and a couple of other circuits.

If so, the 20A fuse in the main board isn't rated for a full 64A load that your subsidiary board is fused for (20 + 32 + 16 + 16). That doesn't seem right to me.

Glossary


MCB - Mini Circuit  Breaker - has an on-off switch that detects overcurrent faults
RCBO - Residual Current Breaker with Overload protection - has a reset pushbutton
GFCI - What Americans call a RCBO (roughly equivalent anyway)
Fuse - Contains a wire that sacrificially melts to protect you when a fault occurs.