Immersion water heater in a metal cup
And a sticker on the cup that says "Be careful not to touch the water or metal cup, while the heater is in use."


A sticker is no substitute for electrical safety. It doesn't protect children or people who can't read or stupid people (who may be smart enough to sue you).

If proper grounding cannot be guaranteed then double-insulated safety category is required. Most water heating elements have a metal outer jacket so you are unlikely to be able to achieve double-insulated safety rating for your device.


  So I think as long as I have a plastic cover, and someone can not the metal or water, it is safe.


Electricity and water are a dangerous combination. You have to think of every possible thing that could go wrong and address it in a risk assessment. From that you modify the design to eliminate the risk. In this case a faulty element or wet cable could be lethal.

The general advice here would be to avoid getting into manufacturing something for sale that might - highly likely in this case - risk harm and you and your charity facing legal action.