Water hose? Maybe...
Before my pressure washer I would park my Toyota 4runner on the grass and stick a fan type grass sprinkler under for awhile. That would get most of the thick crap out, then sit underneath with a hose and brush and go to town. After I got all that, then I would take a rag and armor all and completely go over everything. My toyo was pretty tall so it was easy to do and I really had to because you could see alot of the underside. But she was pretty when I was done.