"De atleten hebben meer water nodig."
Translation:The athletes need to have more water.
There is no need for 'the athletes' "Athletes need more water" is fine!
That's incorrect. In both the Dutch and English sentences, the definite article is used to indicate that its noun is a particular one which is identifiable to the listener, reader, etc. It may be something that the speaker has already mentioned, or it may be something uniquely specified.
Without an article, it would just refer to all athletes in general, which is not the case here.