So I was play testing a monster and it was made of water. The character I was using had the control water spell so I am wondering what would happen if I was to use that spell on a living monster such as a Water Elemental or a Water Weird.
The water in elementals is presumably not freestanding
Ok, this comes down to a reading and meaning of terms the game doesn't define, but control water requires the targeting of freestanding water:
Until the spell ends, you control any freestanding water inside an area you choose that is a cube up to 100 feet on a side.
And if the water is bound up in an elemental or similar, it is not freestanding. For more on that, see: What the heck is "freestanding water"?
As such, the water in the elemental is ineligible as a target for control water.