on ships, water drains off the deck through scuppers. Water flows through an opening on the side, and a little swinging door prevents water from splashing back in just as easily.
on roofs, scupper drains take water off at the side, often through a wall. E.g. http://www.bbsheetmetal.com/downspou...ers/index.html
There are plumbing scupper drains too, floor drain i believe. Shower drains and floor drains are the same thing, in terms of "code". E.g. at http://www.watts.com/pro/_products_s...=67&parCat=373 see rd-270 and rd-290. You would have to clean that horizontal pipe often. Until the P trap.
You can arrange for water to flow into plumbing (P trap) on the outside of the wall. I've never done it, so others can mention caveats and constraints that I won't be able to.