الْمَاءُ قَلِيلٌ.
Water is scarce.
إِنَّ الْمَاءَ قَلِيلٌ.
Water is indeed scarce.
The unmarked report suits a hearer simply receiving information. Inna is a means of strengthening the assertion, especially where doubt or denial makes that reinforcement appropriate. It is not obligatory for every public statement and does not itself supply evidence.
If the dispute is about *how much* water remains, emphasis on the word 'scarce' will not answer the numerical question. Additional measurements or reliable observations would be necessary.