Is taking water from household tanks that belong to no one, or whose owner is traveling, for personal use due to a water cutoff, considered theft? And is it permissible to pay the value of the taken water on behalf of its owner?

There is a contradiction in the question between the tanks being ownerless and their owner being away. If the tanks have an owner, it is not permissible to take water from them because it is his property by possession, and taking it would be considered theft. However, if you are compelled to take the water, you may do so while guaranteeing its value to the owner, because necessity does not nullify the rights of others.