Power steering failure when wet

When what's wet? Do you mean when driving in the rain, through standing water, or there's power steering fluid leaking from something? If the problem is associated with normal rain, sprinkle some water on the serpentine belt while the engine is running to see if that replicates the problem. If it does, check the spring-loaded belt tensioner to be sure it's holding the proper tension on the belt. Also check the pulleys for alignment.