Bhashya
केवलमेव स्वाभाविकमोदनम् ।उदग्ग्रहणम् अन्यप्रसङ्गनिवृत्त्यर्थम् ॥
In plain words
'Water-rice' simply means rice on its own, nothing added. Naming the water is only there to rule out milk, or curds, or anything else creeping in.
Close to the Sanskrit
'[Rice cooked in water]' means rice alone, in its own natural state. The mention of 'water' serves the purpose of excluding association with anything else.