So under the terms of the UDHR it seems that water is deemed to be a right.
Yes
But I guess there remains a discussion over whether this is a 'positive right' or a 'negative right'
A positive right is one where there is an obligation on others (often government) to ensure that the right is fulfilled - in this case to ensure that water is provided, whatever the circumstances.
A negative right is one where there is an obligation not to prevent access to that right. So in this case, not to prevent individuals accessing available water, albeit no obligation to take positive action to ensure they have water.
I would have thought it would be a positive right.
A good example of a 'negative right' is this one from the UDHR:
'Everyone has the right to freedom of thought, conscience and religion; this right includes freedom to change his religion or belief, and freedom, either alone or in community with others and in public or private, to manifest his religion or belief in teaching, practice, worship and observance.'
So a government should not prevent you from practicing your religion, but there is no obligation on the government to build you a church (i.e. a positive right).