Many electric geysers using an element draw in excess of 16 Amps (+-4,000W on a 240V circuit) so using a something like a Sonoff Basic R2 (only rated to a maximum of 10A or 2,200W) would almost certainly overload the Sonoff’s circuitry and cause it to burn out. It is also probably serious fire hazard this is definitely something you should not attempt.
The Sonoff POWR2 on the other hand has a maximum rated load of 15 Amps (3,500W). However, even this switch is probably going to be insufficient to run a geyser element, especially since geyser tend to run at high loads for extended periods of time.
I would only be comfortable pushing a Sonoff switch close to its maximum load/amperage if the high load was only being maintained for a very short period of time. For example running a pump where the starting load id high for a very short period but the running load is well below the maximum.
Are there any alternatives to using a Sonoff switch with a geyser?
No and yes! From what I can see, there are currently no reputable >20A wifi switches on the market. So, from what I can see, there is no way of using a wifi switch to safely control a geyser directly.
That doesn’t mean it cannot be done. I have not tried this myself but apparently you can user a combination of a Sonoff Basic and a 40A contactor to get the job done. From what I understand, the Sonoff Basic is used to switch the 40A contactor which in turn switches the large load to the geyser.
Below is a wiring diagram I found on this Home Assistant Community forum. If you are thinking of trying something like this, PLEASE MAKE SURE you get an electrician to carry out the installation for you as high voltages are involved and there is a potential fire risk from incorrect wiring.