Both terms are used loosely enough that a procurement conversation can run for an hour before anyone notices the two sides mean different things. The distinction is worth being precise about, because one of these is a multi-year capital programme and the other is not.
What a digital twin actually is
A digital twin is a calibrated model of the physical system, connected to live data, kept in step with the asset as it changes. For a water network that usually means a hydraulic model in EPANET or a commercial equivalent, fed by telemetry, with the model updated as mains are replaced and demand shifts.
Its defining property is that it answers "what would happen if". What if we close this main. What if demand rises fifteen percent in this zone. No agent can answer those questions, because the answer is not in the historical data. It comes from simulating physics.
The cost is real: building the model, calibrating it against field measurements, and keeping it current. An uncalibrated twin is a drawing, not a model.
What an agent actually is
An agent reasons over what already exists: data, documents, and tools it can call. It does not simulate. Given a hydraulic model it can run one, which is the useful case, but on its own it answers "what is happening and why" rather than "what would happen if".
The cost profile is different. An agent can be useful in weeks against data a utility already holds, and it degrades gracefully: a poor answer wastes time rather than invalidating a capital asset.
They are not alternatives
The strongest combination is the twin as a tool the agent can call. An operator asks a question in plain language; the agent decides a scenario needs simulating, runs it against the calibrated model, and returns the result with the assumptions stated. The twin supplies the physics, the agent supplies the interface and the triage.
That pairing is what we mean by a decision support system rather than a model: the network model underneath, a decision layer above it, and monitoring keeping both honest.
How to decide what to fund first
If your questions are about futures that have not happened, you need the twin, and you need it calibrated before it is worth anything. If your questions are about a present nobody has time to investigate, the agent gets there sooner and cheaper.
Being sold "an AI digital twin" with no calibration plan and no field measurement campaign is the case to be most careful about. The phrase is doing work the model has not done.