It only sometimes works if the water is constant, and not replenished. And even ...

It only sometimes works if the water is constant, and not replenished. And even then you need an external clock to calibrate each site.

If it's things like floodwater then the errors are too large.

Water does not have the same isotope ratio as air, and water can also move carbon in and out of the sample.

They have procedures for adjusting the dating for marine samples, but it's only valid in certain cases.