" Water is an endless source" would be the better word choice scienctifically speaking because in the 2nd sentence you could be implying that the process of evaporation/condensation (the water cycle) shall never stop (which it can if the sun goes out) and there won't be anymore usable water. Infact both sentence could be saying that. If i may i suggest using a different sentence such as "Water is an endless source if the water cycle continues to run its course" ^^Good line for a poem also, it rhymes
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