"Do they want water?"
Translation:הם רוצים מים?
It is actually the dual form of a very old word. So you cannot add a plural ending. The word מי does exist but would only, as far as I know be found in certain expressions and in place names. For all practical purposes you can treat מים as a plural. Dual words used to be more common in Hebrew. It gets a bit complicated. Some words have occasional dual forms such as day, week, month and year. i.e. If you happen to be talking about two of these, use tbe dual form. Some words naturally come in twos, such as eyes, arms, legs, hands so you normally see these in the dual form but they also have a plural form. Some words such as glasses and bicycle only have a dual form and this is used instead of singular or plural. If you want to say more than one of these words you have to say so many pairs...