Your Home Profile is not completed. Therefore I don't know whether the foundation walls are concrete block or poured concrete, but if I understand correctly there is a significant depression below the deck, and the foundation wall in this area is lower. While I must admit that I don't fully understand your situation (I'm not sure what exterior wall material ison top of the lower section of the foundation wall), you must improve the grading below the deck. This may be possible by removing some deck boards without removing the entire deck. The material below the deck should slope away from the house and should be relatively impervious to water. My suggestion would be to get the appropriate grade, cover the ground with heavy polyethylene,and cover the polyethylene with a couple of inches of gravel, to hold the polyethylene down and protect itfrom any ultraviolet light it might see below the deck. If for some reason my solution is not possible, your only option is to create a low area which flows to a drain. This is obviously less desirable.