You can get 24kW instant electric heaters, provided you can spare a 100A phase for one. I wouldn't attempt to apply diversity given that it will draw the full 100A when operational. Alternatively, I believe you can get 3 phase ones that will only draw 33A off each phase.
These could be a good solution for a shower liking person with lots of electricity and no gas or oil. They would be particularly suitable for a holiday house that is left unattended, as water tanks can be a worry for leakage and accidental heating for long periods (such as over winter).
Christian.