You can water cure bud at any time, dried or not. The water cure is just removing left-over chlorophyll that air-cure does not get out, plus anything else left. Be reminded some of your trichomes might be lost, thus losing potency. This is the sacrifice you make. There IS a point in water curing after you dry, it removes harshness and smell which are useful in some cases.
In order to water cure you must find a way to submerge your buds in water for 5-7 days. This is a task that I thought would be easy, but is not. I favor a wire cage with weights attached but you may have small buds that slip out. A cheap method utilizes large canning jars with holes in the top. Others have tried cylindrical coolers with a dinner plate on top to much success. Whatever your method, make sure you leave the top off so that chlorophyll + water can evaporate. Change the water once a day.