You'd have to water very well, until the ashes were dispersed completely, as water mixed with ash can create lye, which is acidic.
From a magickal stand-point however, it would not be a good idea. You are basically "watering-down" the spell, and from a symbolic perspective it's not good for the spell.
Ashes should be released onto a breeze or allowed to sit out until they are carried off by the wind. Burning the ashes in a completely dry bowl (the shallower the better) is a better idea. Just let the bowl sit out in the open air until they are carried away.