Seeking ideas for powering a hot water cylinder directly from solar panels in an off-grid situation
Thermal panels would be preferale, but since you already have the panels...

If you're looking at a DIY solution, the simplest would be this:

Since the peak of a 240VAC sine is 336V, the heating element's insulation should withstand 336V. Thus you have some flexibility in wiring, as long as you keep the panels' output voltage between, say 200V and 336V.

Next, we need a MPPT, because wiring the resistor directly would produce almost no power in overcast weather, due to the solar panels' I-V characteristic which is more or less a voltage-limited current source, whose current depends on light.

So, I would simply put a capacitor at the output of the panels. It should have a ripple current rating high enough for the power considered here, so something like 15A should do the trick, for example a bunch of big 450V caps in parallel, maybe 1000µF capacitance, value isn't critical.

This is obviously a "kill yourself" project so you have been warned. The panels will also kill you.

Now we have the panels charging the caps, it is a simple matter to do a crude MPPT with a dumb hysteresis comparator: when the voltage on the caps exceeds, say, 250V, turn on the heating with an IGBT or a big MOSFET. When the voltage drops below say, 230V, turn it off.

The idea here is to PWM the load to make it draw an average current that allows the max amount of power to be extracted from the panels. Too much current, and the voltage would drop. The capacitors smooth the current and do the averaging.

Of course you can do a fancy MPPT with a micro if you want to have fun, but a simple bang-bang comparator should work just fine and you'll get hot water on overcast days, it'll just take longer.

EDIT:

You've got me a little worried here:

Hopefully the boiler's heating element should look like that:



This is a resistor, inside a high-temperature insulator, inside a metal pipe which is then earthed. Thus no part of the resistor which is at mains voltage ever comes into contact with the water. Thus no electrolysis takes place...

If the pipe is punctured by corrosion then water will get in and current will flow to Protection Earth, tripping the RCD.

If the mains carrying bits of your heating element are in direct contact with water, this is extremely dangerous. This shouldn't be the case in a boiler, it would be totally insane...

Which reminds me I forgot to recommend tell you to add a RCD to your installation. A quick bit of googling revealed that I have no idea if you can find a RCD which would work on DC. 

So... you have another problem now, which is how to make it safe. Without some kind of protection, if the heating resistor gets corroded you'll have a dangerous situation. Since I'm a landlord as a side job, over the years I've seen this happen once, although the most likely scenario is that the tank itself gets corroded and punctured, and then the water leaks onto the wires, thermostat, etc. In this case the RCD also trips and the tenant calls wondering why they have no power...

I'm not familiar with solar, so I guess the inverters must incorporate such a feature... but if you want to go without an inverter then I guess you should ask another question about how to make this safe :)