Problems with water under the door of a 95 Chevy 1500.?

My brother has a 1995 Chevy 1500 and we just got a lot of rain. He says every time he slows down, and comes to a stop, he hears water gushing somewhere on the passenger side of the truck, He says it is right below the door, in the body. Where most trucks begin to rust out. Is there a possibility that water could have gotten down there, and how the heck would you get it out? Thanks for your time. . .