In my experience I have found most moisture under homes is from surface water.
Surface water is not running away from, but actually toward the home. It only
takes a very small depression to create a very big problem.
Trying to remove the water after it is under the home is a difficult task; you
need to make sure it doesn't get there in the first place.
The first thing to do is to have properly installed rain gutters, the down
spouts should divert the water away from the home, at least 3 feet, and 5 feet
would be better. The second thing to do is to re-grade around the house so water
runs away from the foundation. A 6-inch drop in grade in the first 5 feet
around the house is recommended. The third thing to do is to remove all
plantings around the home and install a 5' membrane around the home and cover
the membrane with the landscape material of your choice.