Arimidex is an estrogen inhibitor. May affect the water retention & estrogen pattern fat depositing related to estrogen, but not water retention due to other things, e.g. non-optimal diet, salt, etc.
Generally I have gathered that it is best to absolutely not artificially manipulate your estrogen levels unless you are very close to a competition date. Otherwise you have to be sooooo careful w/ messign w/ the balance of the hormones in your body. There's more to it than just reducing water bloat....