While a war correspondent

and

While war correspondent

both seem ok. I'm afraid I can't really say which is better Very hard for me as a native speaker to know my own grammar.

I think "a" is probably slightly better if there is no continuation. But I'd put:
"While war correspondent for the BBC". Sorry, can't help much.

I would probably not really write it either way, but:

Hemmingway wrote his most powerful novels (based on his experiences) while he was a war correspondent.

Part in (brackets) can be included, or not.


"The water of nature" is grammatically ok. But "natural water" is a more idiomatic expression meaning nature's water/water of nature/water in nature.

I'd write:

Rain is almost the only pure source of natural water.

Or

Natural water is seldom pure, apart from as rain.