This topic has come up on a few Blogs recently. And I want to add my two cents.
Yes, a wet bottom increases the sting. The more water, the more sting. Even one swat with a paddle removes most of the water, so it needs to constantly be replenished.
If the spanker can squeeze a water mister with one hand and a paddle with the other excellent results can be obtained. If the spanker is not so versatile, then a third party would be handy.
The only drawback to wet spankings is that the spanker is going to get wet, so they may as well undress.
If your backyard is private, a hose running over the target area would be useful. Indoors, a flexible hose from a tub/jacuzzi could be used.
Variations: Have her soak her panties in ice water and spank them dry for her.
Have her sit on a 25 pound block of ice, then paddle her. If she is Canadian, a block of ice would be redundant. <grin>
Always trying to be helpful.