Save the ashes in a bag. Pour the used water in the bag. You'll be amazed how much water those ashes absorb. Chunk the bag in the trash can. Caution: make sure the ashes are dead. You'll also be amazed how long the ashes stay hot and can burn through the bag.
I got this from TVWB:
Dispose Of The Water Pan Waste
Let the water pan cool enough to handle safely before removing it from the cooker. There are several proven methods for disposing of the contents:
Separate the fat from the liquid: Allow the fat to solidified on the surface of the water, skim it off and place it in the trash, and pour the remaining liquid down the sink. The success of this method depends on the type of fat and whether it gets cold enough to solidify.
Bottle the fat and water: Using a funnel and ladle, put the pan contents into a plastic milk jug, cap it, and dispose of it in the garbage.
Bag the fat and water with the cold ashes: Put several plastic bags one inside the other, place the cold ashes in the bag, and then carefully pour the pan contents into the bag. The ashes will absorb the liquid. Tie the bag shut and dispose of it in the garbage. Thanks to Doug D on The Virtual Weber Bullet Board for this idea.
Do not dispose of cooking fat by running it down the drain. This can clog your plumbing and your neighborhood sewer system over time.
Some folks that live in wide open spaces simply dump the pan contents somewhere on their property. This practice can attract a variety of insects and critters that you probably don't want around.
Clean The Water Pan
Soak the pan in hot, soapy water to loosen gunk inside the pan and smoke residue on the outside of the pan. A non-abrasive cleanser like Soft Scrub can also be used. Scrub, rinse, and dry thoroughly.
To make cleanup of the water pan easier, cover it with heavy duty aluminum foil before use. See Using A Water Pan In The WSM for details.
Dispose Of The Ashes Safely
Remove the middle cooking section and set it aside. Remove the charcoal chamber and charcoal grate and brush them off.
Ashes that seem cold on the surface may still be hot deep inside, even after sitting for two days, so take care when handling and disposing of them.
The safest way to dispose of ashes is to put them in a small galvanized metal trash can that is dedicated to the purpose of ash storage. Keep the can away from any flammable materials, including your house, wood pile, dry grass, or weeds. Let the ashes sit for a good, long time until there is absolutely no doubt they are fully extinguished, then dispose of them in your regular household garbage.
If a dedicated ash can is not an option, stir through the ashes to make sure they are absolutely cold before placing them in the garbage.