Easy

You are responsible.

The tenant with no water has recourse against the landlord.

When a landlord take money in return for property, they give what is called an "implied warranty of habitability". Basically, they guarantee that the property will be livable.

In this case, the tenant who has been deprived water can sue you for the decrease in value of the property based on the actions of the other tenant.

So what can you do?

You clearly have a claim against the tenant who caused this mess. IF they will not fix the problem, you can evict them. AND you can sue them for any money you loose because of this.

YOu would need to give them notice first, in writing, and time to fix. But what you describe? It should not take long to fix

And as the landlord, if you retained keys, you can access the property and make the changes yourself.

But I would start with notice to the tenant and demand they fix the problem, letting them know if they refuse you will evict them. Then if they refuse? Head to the county court and file an eviction notice to start the process to evict and if you can, enter the property and correct the problem.

If there is any indication that the vindictive tenant may become violent, you can go to court and ask for a temporary restraining order...this is something that can happen very quickly. But only if there is indication of threat to safety.