Crossy is on track in asking for the Kw rating of the heater and> is it the blue mains breaker? or the red Earth leakage C/B? and> if its the main breaker it could be hot from nearly overload and/or> tripping early being faulty. But consider this > If the water heater is perhaps 6kw then the current would be peaking/cycling at 25 Amps (some are as high as 6,8kw rating) and hopefully the house wiring being used is not just typical 10Amp, and youve also got some pretty serious C/B on the right so the total amps could be stretching the friendship on the 63 Amp blue mains breaker, even if its not faulty to be "tripping early". The very nature of the water heater element construction may have small electrical leakage and be intermittently tripping the red one?, Perhaps not since you havent reassured us about which one> Blue/red?
If your water heater model is excessive in power rating?, consider replacing with as low as a 1.8 kw model? especially for being kind to the house wiring as well as "avoiding" the tripping. (see below.) If the installed unit is too high powered, even though the temperature setting is just right for you, it is surely cycling full on then off etc, and the on cycle means a high a current and could be the culprit adding to when the a/cons are possibly cycling on/off too. Regardless of their nominal Kw rating, the start current of their compressors are typically 6 times (momentarily) their running current, and adding them up (when they coincide) will surely trip the blue mains C/B. Be aware about the A/Con stated ratings. Sure there is the electrical k/w rating, but they often proudly boast the Kw heat/cool rating (like a BTU rating) which is about 4 times the actual electrical rating because of the COP. Coefficient Of Performnce, i.e. the efficient ability of evaporation of the refrigerant to transfer British Thermal Units of energy from outside to inside of house or viceverca. The electrical k/w rating is the go but there are the peaks as above.
Hope this helps your chosen, thinking, electrician and do let us know red or blue, and the size of the water heater hey.