Ruptured water heater

Hertz matters for motors, processors. A heating element wouldn't care. And it's not the voltages per se, it's how it's wired. I THINK a heating element wouldn't know the difference. Hence the ability of PO/yard to be creative in the engineering.

That said, a thermostatic control would have controlled heat in the thing regardless and if it didn't, Capt Ralph is on the money, the blow off should have done its thing. This sounds like a compound failure. You're lucky it didn't blow. I've blown up water systems, not pretty.

Oh yeah. Eve more impressive than knocking the valve off a gas bottle....

100% agreed.
And even if I can't be positive about this, I believe that the above type of heater is designed to be equipped with an external expansion vase, and THAT should incorporate a pressure safety valve, which I don't think exists at all in the heater itself.
Besides, in the second photo of the OP there is one part missing, i.e. the stem thermostat that goes inside the hole, and that allows to regulate the working temperature.
This is the producer webpage of the missing component, for your reference.

FWIW, I like to keep that as low as 50°C (around 120F) on my boat, which doesn't stress the expansion vase at all, but that depends to some extent also on the heater capacity vs. the onboard needs.
In fact, another pet peeve of mine is the heating through engines, which I hate.
My previous boat was equipped with a very basic heater, actually meant for home usage, and it's still working just fine after a quarter of a century.
My current boat came instead with a nautical heater similar to the one which is being discussed, engines circulation included, and the first thing I did when I bought her was to close the circulation valves on the engine side.
Aside from the fact that also while cruising I never stay without AC onboard for long periods of time, the engines cooling liquid flows at around 85°C (185F), so it gets also the fresh water equally hot inside the heater tank, regardless of any regulation of the electrical element.
And I neither need nor want it to get so hot.

But of course, in principle the heater is designed to handle such high temperature, so I'm not excusing at all the fact that it didn't withstand it - aside from being maybe not installed properly, with no safety valve.

This was not caused by electrical voltage ,watts or frequency. Steam is compressible, water is not. For this to happen , 1: There is not a properly working expansion tank on the hot side of domestic water. 2: There is not a properly working pressure relief valve.... (many times a relief valve is Dripping or blowing off and someone decides to fix it with a plug. ) By doing that someone created a bomb. That was very close to a catastrophic failure.

As Mapism pointed out, there was no thermostat control of the water temperature. Previous owner/paid hacker removed it from the circuit? Without a thermostat, the pressure relief would weep/vent with some regularity because the element was on continuously. Same hacker plugged that "leak" by disabling the relief valve?

A 1500W @220V element would only provide 375W @110V (power varies with the square of the voltage), so maybe the tank construction could live with that heating energy until it couldn't anymore.

A hot side expansion tank is a great idea and will keep the heater relief valve from weeping, but, IMHO, most boats don't have them (mine does now).

I don't think the problem(s) was the fault of the manufacturer - good news is - you are back in business, anyway.