Does anyone know this Chinese poem?
The poem you are quoting is 前赤壁赋 by 苏轼，and indeed, it is one of the best 赋 in Chinese history.

The part you have in the post is


  客亦知夫水与月乎？逝者如斯，而未尝往也，盈虚者如彼，而卒莫消长也。盖将自其变者而观之，则天地曾不能以一瞬，自其不变者而观之，则物与我皆无尽也，而又何羡乎？且夫天地之间，物各有主，苟非吾之所有，虽一毫而莫取。惟江上之清风，与山间之明月，耳得之而为声，目遇之而成色，取之无禁，用之不竭，是造物者无尽臧也，而吾与子之所共适。