In the half-reaction 2 H₂O → O₂ + 4 H⁺ + 4 e⁻, water acts as:

1. O goes from −2 (in H₂O) to 0 (in O₂) — that's an INCREASE in oxidation state ⇒ oxidation. 2. Whatever is oxidised loses electrons and is a REDUCING agent. 3. Hence H₂O is acting as a reducing agent in this half-reaction. 4. (In photosynthesis, water is similarly oxidised to O₂.) _Source: syllabus Chem Ch 8 §8.2 Identifying Redox Pairs_